[Mac] Use pageScaleFactor * deviceScaleFactor in requiresTiledLayer() and computePixelAlignment() of GraphicsLayerCA.
https://bugs.webkit.org/show_bug.cgi?id=107359

Patch by Huang Dongsung <luxtella@company100.net> on 2013-05-13
Reviewed by Darin Adler.

Currently GraphicsLayerCA uses pageScaleFactor in requiresTiledLayer() and
computePixelAlignment(), but we must use pageScaleFactor * deviceScaleFactor in
them.
It is because:
1. requiresTiledLayer() uses the scale to compute an actual layer size in the device
pixel unit.
2. computePixelAlignment() uses the scale to compute an aligned layer position
in the device pixel unit.

No new tests. We can not test about requiresTiledLayer() because it depends on
gpu. computePixelAlignment() is correct now because fortunately Mac uses
only 2 (for retina display) as a deviceScaleFactor.

* platform/graphics/ca/GraphicsLayerCA.cpp:
(WebCore::GraphicsLayerCA::requiresTiledLayer):
(WebCore::GraphicsLayerCA::computePixelAlignment):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150047 268f45cc-cd09-0410-ab3c-d52691b4dbfc