Merge 130322 - [Sub-pixel layout] incorrect rendering when painting sub-layers as their own root

Merge 130322 - [Sub-pixel layout] incorrect rendering when painting sub-layers as their own root
https://bugs.webkit.org/show_bug.cgi?id=97484

Reviewed by Eric Seidel.

When in compositing mode, layer painting can be triggered through the backing store. When this
happens, a non-top-level RenderLayer is called to paint as its own root. Normally, we attempt to preserve
the proper sub-pixel accumulation through layers to their children, but since we're not
starting with the top-level layer, we haven't properly accumulated one, and convertToLayerCoords,
another source of correctly getting the sub-pixel offset for a layer, also avoids crawling
past the listed root layer.

When painting a root layer, we're aligned to the surface we're painting to, so we round our
offset to avoid moving objects around.

* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::paintLayerContents):


TBR=leviw@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=130526