Fix border painting on self-painting layers.

Fix border painting on self-painting layers.

Commit efc3afd428bd85fd3d12e0dc941b5eb7248ca30b broke it because
we'd computed an empty foreground layer. Now that background
painting is part of the foreground phase, we need to use the
background rect for clipping. As best I can tell, the
background rect is always >= the size of the foreground
rect, so it should be safe to use.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/548e122965df9d8572e7df2b1064d920528d8248

[ojan, 2015-01-27 06:13:41]

ojan@chromium.org changed reviewers:
+ abarth@chromium.org, eseidel@chromium.org, esprehn@chromium.org

[abarth-chromium, 2015-01-27 06:31:05]

lgtm

[ojan, 2015-01-27 06:45:23]

https://codereview.chromium.org/876243002/diff/20001/sky/tests/framework/flig...
File sky/tests/framework/flights-app-expected.txt (right):

https://codereview.chromium.org/876243002/diff/20001/sky/tests/framework/flig...
sky/tests/framework/flights-app-expected.txt:141: layer at (609,509) size 159x59
clip at (610,510) size 157x57
Had to at this rebaseline to the patch. It's from the overflow:hidden added to
the test, not from the C++ change.

[ojan, 2015-01-27 06:45:34]

Committed patchset #2 (id:20001) manually as
548e122965df9d8572e7df2b1064d920528d8248 (presubmit successful).