CrOS Shelf: Remove overflow shelf clip

Remove overflow shelf clip so that inkdrop ripples can propagate
outside the overflow bubble.

The mask isn't really useful in the first place anyway.

See before/after screenshots here:

    https://bugs.chromium.org/p/chromium/issues/detail?id=631240#c14

Bug: 631240
Change-Id: Iaa1d2f4e2e74bff34a708fa9e2cd7be5ed9a52fa
Reviewed-on: https://chromium-review.googlesource.com/c/1429639
Auto-Submit: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625234}
1 file changed