https://bugs.webkit.org/show_bug.cgi?id=124004

Reviewed by Tim Horton.

On some platorms, zooming out on pages with TiledBacking compositing
layers can cause very high memory use, because the TiledBacking retains
the original page scale while the zoom is in flight, but can be asked
to cover a large area.

Make it possible to reduce memory use in this case by allowing RenderLayerCompositor
to provide an additional scale factor for newly created tiles. Platforms can
then customize this to create low-res tiles when necessary.

* WebCore.exp.in:
* platform/graphics/GraphicsLayerClient.h:
(WebCore::GraphicsLayerClient::contentsScaleMultiplierForNewTiles):
* platform/graphics/ca/GraphicsLayerCA.cpp:
(WebCore::GraphicsLayerCA::platformCALayerContentsScaleMultiplierForNewTiles):
* platform/graphics/ca/GraphicsLayerCA.h:
* platform/graphics/ca/PlatformCALayerClient.h:
(WebCore::PlatformCALayerClient::platformCALayerContentsScaleMultiplierForNewTiles):
* platform/graphics/ca/mac/TileController.h:
* platform/graphics/ca/mac/TileController.mm:
(WebCore::TileController::TileController):
(WebCore::TileController::setScale):
(WebCore::TileController::createTileLayer):
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::contentsScaleMultiplierForNewTiles):
* rendering/RenderLayerBacking.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::contentsScaleMultiplierForNewTiles):
* rendering/RenderLayerCompositor.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@158859 268f45cc-cd09-0410-ab3c-d52691b4dbfc