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This CL makes the top corners of BottomSheet round shape (of radius
4dp). The round corners resides inside the top shadow and takes
the half the height.

Bug: 985644
Change-Id: Ibfc26c06a9ca4c83aa5f7304e27a91f50fba9df7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710016
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680342}