Make GraphicsLayerCA::backingStoreMemoryEstimate() give a better estimate for tiled layers
https://bugs.webkit.org/show_bug.cgi?id=112864

Reviewed by Tim Horton.

If a GraphicsLayerCA has TiledBacking, ask the TiledBacking how
much memory it's using for tiles, rather than just using
height * width * 4.

Implement retainedTileBackingStoreMemory() in the TileController,
summing the memory used by parented tiles.

The original code was incorrectly multiplying by contentsScale
only once rather than twice (2x contents scale uses 4x as much memory).

* platform/graphics/TiledBacking.h:
* platform/graphics/ca/GraphicsLayerCA.cpp:
(WebCore::GraphicsLayerCA::backingStoreMemoryEstimate):
* platform/graphics/ca/mac/TileController.h:
* platform/graphics/ca/mac/TileController.mm:
(WebCore::TileController::retainedTileBackingStoreMemory):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146438 268f45cc-cd09-0410-ab3c-d52691b4dbfc