[chromium] Only apply page scale delta to root scroll layer

[chromium] Only apply page scale delta to root scroll layer
https://bugs.webkit.org/show_bug.cgi?id=91374

Patch by Sami Kyostila <skyostil@chromium.org> on 2012-07-16
Reviewed by Adrienne Walker.

Source/WebCore:

When the user pinch-zooms the web page though the Chromium compositor, the
per-layer page scale delta is used to keep track of the difference between the
page scale on the compositor thread versus the main thread. On the next
commit to the main thread these values are reset to 1.

When calculating layer positions, the compositor applies a layer's page scale
delta both to the layer itself as well as all of its children. Since we are
currently updating the page scale delta on all scrollable layers, this results
in scrollable child layers getting scaled multiple times.

This patch changes the compositor to only apply the page scale delta on the
root scroll layer.

New unit test: CCLayerTreeHostImplTest.pageScaleDeltaAppliedToRootScrollLayerOnly

* platform/graphics/chromium/cc/CCLayerTreeHostImpl.cpp:
(WebCore::CCLayerTreeHostImpl::setPageScaleFactorAndLimits):
(WebCore::CCLayerTreeHostImpl::setPageScaleDelta):

Source/WebKit/chromium:

New unit test
CCLayerTreeHostImplTest.pageScaleDeltaAppliedToRootScrollLayerOnly to verify
the transformation of child layer while pinch zooming.

* tests/CCLayerTreeHostImplTest.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@122746 268f45cc-cd09-0410-ab3c-d52691b4dbfc