Adds public-facing documentation about recent changes to offscreen.

Includes 'invisible' and size/location/clipping as well.

Change-Id: I0841060a41d7d4cff823a309bc7cde6cfc9d46ec
Reviewed-on: https://chromium-review.googlesource.com/797898
Commit-Queue: Katie D <katie@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521373}
2 files changed