[cc] Apply sublayer transform about anchor instead of about layer center.

[cc] Apply sublayer transform about anchor instead of about layer center.

There are some cases where WebCore internally needs to "reposition" the origin
point about which CSS transforms are applied. For example, when a perspective
container becomes composited, the composited location/bounds may change
depending on the positioning of its children. The perspective-origin from the
original DOM tree should not change, but internally it does require a new
origin to be given to the compositor. It turns out that WebCore uses the anchor
point layer property to communicate these cases. So, the compositor needs to
apply the sublayer transform about the anchor point instead of about the layer
center.

BUG=157961
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=181957

[shawnsingh, 2013-02-12 00:53:43]

PTAL - thanks in advance.

[danakj, 2013-02-12 00:58:23]

Assuming this doesn't break the web, the changes LGTM

https://codereview.chromium.org/12224113/diff/1/cc/layer_tree_host_common.cc
File cc/layer_tree_host_common.cc (right):

https://codereview.chromium.org/12224113/diff/1/cc/layer_tree_host_common.cc#...
cc/layer_tree_host_common.cc:870: // Apply the sublayer transform at the center
of the layer.
This comment is wrong now.

[shawnsingh, 2013-02-12 01:05:44]

Comments fixed in latest patch.

Manual testing of many 3d sites looks just fine.  Unit tests all pass, and
layout tests in compositing/ pass too.

[enne (OOO), 2013-02-12 01:07:23]

lgtm2

[commit-bot: I haz the power, 2013-02-12 16:53:29]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/shawnsingh@chromium.org/12224113/4

[commit-bot: I haz the power, 2013-02-12 18:31:23]

Change committed as 181957