cc: Don't clear transparent piles to an opaque color

cc: Don't clear transparent piles to an opaque color

The pile background color is what the pile gets cleared to to fill
partially rastered pixels.  If the layer is not opaque, clearing to an
opaque color leaves opaque artifacts.

Fix this by adding a function to return a safe color to clear the pile
to or to draw solid color quads from by requiring that this color be
opaque iff the contents of the layer are opaque.

R=danakj@chromium.org
BUG=245624
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=204048

[danakj, 2013-06-04 14:34:44]

LGTM

We need a LayerCommon class or something.

[commit-bot: I haz the power, 2013-06-04 15:33:21]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/enne@chromium.org/15995031/1

[commit-bot: I haz the power, 2013-06-04 21:14:51]

Change committed as 204048