WebLayer: adjust Web Contents size when fullscreened and OSK shows

When in fullscreen, the soft keyboard doesn't adjust the window size,
so we need extra logic (copied from Chrome) to adjust the web contents
size in this case.

Bug: 1107651
Change-Id: Ieded2ca098cf4dd5ed210e2043f1b94667579071
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2313979


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791462}