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This patch adds a mask layer to generate gradient fade effect on the media
string.

Bug: b/167236615
Test: added new tests
Change-Id: I4c812c678031d0680c039c2c629aca082550ca9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414670
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810934}