clarify minimum-scale expansion for ICB vs. layout viewport

[skobes-chromium]

The ICB explainer currently suggests that the ICB matches the size of the layout viewport, modulo UA UI shrinkage. That's not really correct, at least for mobile Chrome it is easy to reproduce a layout viewport much larger than the ICB:

https://output.jsbin.com/meyexut/quiet

A clearer statement would be:

• The ICB is sized to match the visual viewport at pinch-zoom scale == 1.0
• The layout viewport is sized to match the visual viewport at the minimum pinch-zoom scale

The minimum pinch-zoom scale is influenced by two things:

• the minimum-scale parameter in the meta viewport tag
• the extent of content overflow (zooming out can't reveal space that isn't occupied by content)

The minimum-scale default is really small and it's pretty common to have horizontal overflow from fixed-width elements, so you actually see LV >> ICB frequently on the web.

[skobes-chromium]

Oops my "clearer statement" is also wrong.

• The ICB is sized to match the visual viewport at pinch-zoom scale == 1.0

^ this is only true when the meta viewport has width=device-width. Otherwise it's sized to the declared meta viewport width, or to 980px if there is no meta viewport tag.

[skobes-chromium]

I made a draft of a doc that tries to explain all of this:

https://docs.google.com/document/d/1FoAGu2grMAnUDgEfLAvALsdDTekstpxb-eeGUhCYTUk/edit

I'll try to make some PRs to update the explainers in this repo.