Revert "CrOS Shelf: Remove overflow shelf clip"

This reverts commit 0cee3055.

Reason for revert: This change makes app icons bleed outside of the overflow when there are too many of them.

Original change's description:
> 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}

TBR=xiyuan@chromium.org,manucornet@chromium.org

Change-Id: I51a004caf0a1b516fd21251621e9f716a241ca14
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 631240
Reviewed-on: https://chromium-review.googlesource.com/c/1432732Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625407}

ash/shelf/overflow_bubble_view.cc

@@ -60,8 +60,10 @@ OverflowBubbleView::OverflowBubbleView(ShelfView* shelf_view,
   else
     set_margins(gfx::Insets(kEndPadding, 0));
 
+  // Makes bubble view has a layer and clip its children layers.
   SetPaintToLayer();
   layer()->SetFillsBoundsOpaquely(false);
+  layer()->SetMasksToBounds(true);
 
   CreateBubble();