GitLab

When in tablet mode on Chrome OS, webpages should have a
mobile-like behavior such as:
- Shrink viewport contents to fit the window size.
- Mobile viewport style.

This also exposes WebContents::NotifyPreferencesChanged() so
that it's possible to trigger an update of all RVHs associated
with a WebContents including OOPIFs

BUG=822455,828697
TEST=browser_tests --gtest_filter=TabletModePageBehaviorTest.*

Change-Id: Id80316fdb7c504a8a6f40ca9127c13ca58a9eee9
Reviewed-on: https://chromium-review.googlesource.com/978519
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548104}