Add Region::Area.

This will be used to measure layout jank regions.

Also add some documentation of Region::Shape, and move comments about
Region into the header.

Bug: 581518
Change-Id: I54365b4fa544210060cc8fa59f4d4af820bfcb09
Reviewed-on: https://chromium-review.googlesource.com/1055597
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560265}
3 files changed