Keep track of the number of GraphicsLayers with tiled backing in RenderLayerCompositor
https://bugs.webkit.org/show_bug.cgi?id=112083

Reviewed by Dean Jackson.

Have RenderLayerCompositor keep track of how many of its GraphicsLayers use TiledBacking.
This is useful for platforms to decide whether it's necessary to force GraphicsLayer flushes
to update the visibleRect of those TiledBackings.

GraphicsLayers are always created without TiledBacking. There are two
ways the count can change:
1. A GraphicsLayerCA toggles into or out of tiled mode. It notifies its
   client via a new tiledBackingUsageChanged() callback on GraphicsLayerClient.
2. A tiled GraphicsLayerCA is destroyed. Added a new bottleneck function to
   RenderLayerBacking::willDestroyLayer() that is called everywhere we destroy layers.

The main page TiledBacking is not counted as part of this.

Also renamed uses of "tiledLayer" to "tiledBacking" in GraphicsLayer[CA].

* platform/graphics/GraphicsLayer.cpp:
(WebCore::GraphicsLayer::GraphicsLayer):
(WebCore::GraphicsLayer::getDebugBorderInfo):
(WebCore::GraphicsLayer::dumpProperties):
* platform/graphics/GraphicsLayer.h:
(WebCore::GraphicsLayer::usingTiledBacking):
* platform/graphics/GraphicsLayerClient.h:
(WebCore::GraphicsLayerClient::tiledBackingUsageChanged):
* platform/graphics/ca/GraphicsLayerCA.cpp:
(WebCore::GraphicsLayerCA::recursiveCommitChanges):
(WebCore::GraphicsLayerCA::updateGeometry):
(WebCore::GraphicsLayerCA::updateLayerDrawsContent):
(WebCore::GraphicsLayerCA::updateContentsScale):
(WebCore::GraphicsLayerCA::swapFromOrToTiledLayer):
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::willDestroyLayer):
(WebCore::RenderLayerBacking::tiledBackingUsageChanged):
(WebCore::RenderLayerBacking::destroyGraphicsLayers):
(WebCore::RenderLayerBacking::updateClippingLayers):
(WebCore::RenderLayerBacking::updateOverflowControlsLayers):
(WebCore::RenderLayerBacking::updateForegroundLayer):
(WebCore::RenderLayerBacking::updateBackgroundLayer):
(WebCore::RenderLayerBacking::updateMaskLayer):
(WebCore::RenderLayerBacking::updateScrollingLayers):
(WebCore::RenderLayerBacking::compositingLayerType):
* rendering/RenderLayerBacking.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::RenderLayerCompositor):
(WebCore::RenderLayerCompositor::layerTiledBackingUsageChanged):
* rendering/RenderLayerCompositor.h:
(WebCore::RenderLayerCompositor::haveNonMainLayersWithTiledBacking):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145563 268f45cc-cd09-0410-ab3c-d52691b4dbfc