[PE] Fix DCHECK failure in scrolling background visual rect

Now use the same pixel snapping method for scrolling contents layer
used in CompositeLayerMapping to avoid the DCHECK failure. The scrolling
contents should originate from the pixel snapped clip rect.

Bug: 913843
Change-Id: Ibce87b324567f971516148f865608f00e80d4bee
Reviewed-on: https://chromium-review.googlesource.com/c/1378850
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616909}
2 files changed