https://bugs.webkit.org/show_bug.cgi?id=74369

Reviewed by Anders Carlsson.

Some platforms may wish to have the main root layer (which corresponds with
the main frame's RenderView's layer) implemented by a custom platform layer,
for example to contain a cache of tiles.

Make this possible on Mac by adding a new method to GraphicsLayerClient(),
and implementing it in RenderLayerBacking. This new behavior is not yet enabled.

Also clean up some WebLayer/WebTiledLayer code.

* WebCore.xcodeproj/project.pbxproj:
* platform/graphics/GraphicsLayerClient.h:
(WebCore::GraphicsLayerClient::shouldUseTileCache): New client method that indicates that the GraphicsLayer
should host a tile cache layer instead of a normal layer.
* platform/graphics/ca/GraphicsLayerCA.cpp:
(WebCore::GraphicsLayerCA::GraphicsLayerCA): Ask the client, if any, whether to create a tile cache layer.
* platform/graphics/ca/PlatformCALayer.h:
* platform/graphics/ca/mac/PlatformCALayerMac.mm: NSClassFromString(@"CATransformLayer") was for Leopard;
we can just use the classname now. Handle LayerTypeTileCacheLayer.
(PlatformCALayer::PlatformCALayer):
* platform/graphics/ca/mac/WebTileCacheLayer.h: Copied from Source/WebCore/platform/graphics/mac/WebLayer.h.
* platform/graphics/ca/mac/WebTileCacheLayer.mm: Copied from Source/WebCore/platform/graphics/mac/WebLayer.h.
* platform/graphics/mac/WebLayer.h: Remove the setLayerNeedsDisplayInRect() hackery.
* platform/graphics/mac/WebLayer.mm: setLayerNeedsDisplayInRect() tried to share code between WebLayer and WebTiledLayer
by using Obj-C runtime methods to find the superclass. This causes infinite recursion if Web[Tiled]Layer is subclassed,
so remove it.
(-[WebLayer setNeedsDisplayInRect:]): Code moved here from setLayerNeedsDisplayInRect.
* platform/graphics/mac/WebTiledLayer.mm:
(-[WebTiledLayer setNeedsDisplayInRect:]): Code copied here from setLayerNeedsDisplayInRect.
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::RenderLayerBacking): Find out, and cache if we're the main frame's layer.
(WebCore::RenderLayerBacking::shouldUseTileCache): Return m_usingTiledCacheLayer, which is always false for now.
(WebCore::RenderLayerBacking::createPrimaryGraphicsLayer): Use m_isMainFrameRenderViewLayer.
(WebCore::RenderLayerBacking::paintingGoesToWindow): The tile cache layer needs to paint itself.
* rendering/RenderLayerBacking.h:
* rendering/RenderLayerCompositor.h: m_compositeForFixedPosition was unused.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@102644 268f45cc-cd09-0410-ab3c-d52691b4dbfc