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Add the gradient zone to the scrollable shelf. The zone's location
is based on the layout strategy according to the new spec.

Bug: 973481
Change-Id: I2949ca70bea6219dd2df3a7cf377050f5cb8b3fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764765
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690144}