SkCanvas::saveLayer(Alpha) which is indirectly generated by
CompositingDisplayItem takes an optional bounds as a hint for the size
of the backing that is used as an indirect offscreen texture for the
saveLayer.  This acts as an implicit clip, but Skia is free to make
this as large as possible or drop the clip.  PaintOpBuffer in particular
has some optimizations that sometimes drop the bounds when trying to
fold together saveLayer/draw/restore into a single draw with alpha.

This patch makes it so that any CompositingDisplayItem that provides
a clip has its content explicitly clipped.  This prevents optimizations
inside of PaintOpBuffer that would remove the save/restore generated
by the CompositingDisplayItem but prevents any accidental painting
outside of display item bounds.

Bug: 750252
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ic651c35e1ce865f3008dd345952a6fbc97d94740
Reviewed-on: https://chromium-review.googlesource.com/596503


Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491915}