Round off high-resolution timers

[arthuredelstein]

Includes rounding of DOMHighResTimeStamps and other timers with resolution higher than 1 ms.

Behind "BraveRoundTimeStamps" feature flag, enabled by default.

Resolves brave/brave-browser#24681

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[goodov]

please rebase to latest master.

[goodov]

lgtm with few nits.

[outermeasure]

Why was privacy/security review skipped since this is a security feature? Does it actually increase security?

See discussion thread: w3c/hr-time#79
The official test for spectre is: https://wpt.live/hr-time/timing-attack.html, and Brave passes without the 1ms rounding.

The excessive rounding makes JS animations (ones that use requestAnimationFrame) janky and WebGL based browser games or apps unviable. I recommend going to 100 microseconds coarse resolution to be the same as Chrome.

Edit: Rounding to an integer is not helpful because an attacker can just repeat the timed operation 10^N times and find N decimals from performance.now().

[RazvanCalin13]

Why was privacy/security review skipped since this is a security feature? Does it actually increase security?

See discussion thread: w3c/hr-time#79 The official test for spectre is: https://wpt.live/hr-time/timing-attack.html, and Brave passes without the 1ms rounding.

The excessive rounding makes JS animations (ones that use requestAnimationFrame) janky and WebGL based browser games or apps unviable. I recommend going to 100 microseconds coarse resolution to be the same as Chrome.

Edit: Rounding to an integer is not helpful because an attacker can just repeat the timed operation 10^N times and find N decimals from performance.now().

Sounds like a solid reason to be dropping 'Brave' from the roster :)

[goodov]

Why was privacy/security review skipped since this is a security feature? Does it actually increase security?

See discussion thread: w3c/hr-time#79 The official test for spectre is: https://wpt.live/hr-time/timing-attack.html, and Brave passes without the 1ms rounding.

The excessive rounding makes JS animations (ones that use requestAnimationFrame) janky and WebGL based browser games or apps unviable. I recommend going to 100 microseconds coarse resolution to be the same as Chrome.

Edit: Rounding to an integer is not helpful because an attacker can just repeat the timed operation 10^N times and find N decimals from performance.now().

@outermeasure
Can you share some examples please? We're trying to do the same thing Safari does, but maybe we're missing something. A simple https://codesandbox.io/ sketch with a clear reproduction would be amazing.

[bridiver]

you have a RenderViewTest in browser. This belongs in renderer