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Before this patch and the one that was the proximate cause of issue 674098,
clips for composited bounds worked like this:
1. If there was a clip between the compositing container and a PaintLayer, the
clip would be that PaintLayer's contribution to composited bounds.

2.  If there was no such clip, we would fall back to physicalBoundingBox, and
also recurse into children.

After the patch causing issue 674098, we would always recurse, even in case 1.

This patch observes that we can always use a local bound of the intersection of
any clip with physicalBoundingBox. Recursion still always happens.

This avoids situations like in issue 674098 where skew matrices yielded bounds
much too large.

BUG=674098
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2591883002
Cr-Commit-Position: refs/heads/master@{#440929}