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Previously the painter for our mask layer was a round rect with all
corners of the same corner radius. This caused us to clip content that
appeared in the upper right and left corners.

Now, the painter for our mask layer only rounds the lower right and left
corners to avoid painting outside container bounds. This prevents content
in the upper corners of our contents from being clipped.

See bug for before/after.

Bug: b:116626523
Change-Id: I5295a4ec22217aa35c973a5f8fc29fee47a1d1af
Reviewed-on: https://chromium-review.googlesource.com/1244337Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#594531}