ui/cc minor doc updates

Updates ui::Layer::SetMasksToBounds() to indicate masking also applies to
this layer.

Updates how_to reference in cc/README.md to point to the updated place.

BUG=none
TEST=none

Change-Id: I0f94a91afc3d8260723afde62a51de6561c245c7
Reviewed-on: https://chromium-review.googlesource.com/c/1497634
Auto-Submit: Scott Violet <sky@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637143}
2 files changed