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Layer::Clone(), which may be triggered by screen rotation, does not
copy the mask layer. As a result, gradient zone for scrollable
shelf may not show after screen rotation. This CL fixes the issue
by checking the mask layer in ScrollableShelfView::Layout() and
setting the mask layer if necessary.

Bug: 1035715
Change-Id: I0f8adb1df9991066c2e6ff46a401a561fdb15204
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986055
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728326}