Provides stub implementation for RouteObserverMixIn

[sangwoo108]

RouteObserverMixin's currentRoutChanged should be overridden. Seems to be regressed from #13290

shared.rollup.js:424 Uncaught Error: Assertion failed: Unreachable code hit
    at assert (shared.rollup.js:424:147)
    at assertNotReached (shared.rollup.js:424:403)
    at SettingsBraveTorPageElement.currentRouteChanged (shared.rollup.js:425:4875)
    at shared.rollup.js:425:1844
    at Set.forEach (<anonymous>)
    at Router.setCurrentRoute (shared.rollup.js:425:1815)
    at Router.navigateTo (shared.rollup.js:425:3177)
    at SettingsMenuElement.onSelectorActivate_ (settings.js:5314:1147)
    at i.<anonymous> (polymer_bundled.min.js:1:23231)
    at i.fire (polymer_bundled.min.js:1:84941)

https://github.com/chromium/chromium/blob/646d3dcc08ed74d1b9522f15270251318bab15ae/chrome/browser/resources/settings/router.js#L434

Resolves brave/brave-browser#24510

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:

[sangwoo108]

@boocmp Thanks for the review! :)

[boocmp]

@boocmp Thanks for the review! :)

@sangwoo108 I was sure I added this override, but it seems I accidentally deleted it :(