Merge 120640 - [chromium] REGRESSION(r120346) ContentLayerPainter should clear rect to be painted
Reviewed by Adrienne Walker.
In r120346, a refactor moved a clearRect()/clip() call from ContentLayerPainter - which all
ContentLayerChromiums use - out to OpaqueRectTrackingContentLayerDelegate - which all GraphicsLayerChromium
backed layers use, but which callers using WebContentLayer directly do not use. This meant that the canvas
wasn't cleared/clipped as WebContentLayerClient.h claimed it would be. This moves the clear and the clip
into CanvasLayerTextureUpdater which all paths use, including layers that don't use the ContentLayerPainter.