Enable inert visual viewport

Inert visual viewport is a mode in Blink that makes the visual
viewport invisible to the page in most ways. That is, pinch-zooming
in and panning the visual viewport around doesn't affect window's
innerWidth/innerHeight, scrollX/scrollY properties. It also makes
scroll methods like scrollIntoView work on the layout viewport.

For all intents and purposes, the web page behaves as if the browser
is fully zoomed out at all times.

Intent-to-Ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/kbcFPnvDwUc/P2PLzcGhBAAJ

Bug: 489206
Change-Id: Ic115960dc71df9bf5885e7bfe42692cbab01704b
Reviewed-on: https://chromium-review.googlesource.com/545143
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482525}
2 files changed