Blink API to allow mailbox usage with texture layers for WebGL and Canvas2D.

Source/Platform/chromium/public/WebCompositorSupport.h

Index: Source/Platform/chromium/public/WebCompositorSupport.h
diff --git a/Source/Platform/chromium/public/WebCompositorSupport.h b/Source/Platform/chromium/public/WebCompositorSupport.h
index 6b60a9e91ff4eb8d9f023713a62fb2171a309aac..53cb3b7c9a4a05eaebad55c293ca165287965dfd 100644
--- a/Source/Platform/chromium/public/WebCompositorSupport.h
+++ b/Source/Platform/chromium/public/WebCompositorSupport.h
@@ -77,6 +77,8 @@ public:
 
     virtual WebExternalTextureLayer* createExternalTextureLayer(WebExternalTextureLayerClient* = 0) { return 0; }
 
+    virtual WebExternalTextureLayer* createExternalTextureLayerForMailbox(WebExternalTextureLayerClient*) { return 0; }

[Ken Russell (switch to Gerrit), 2013/04/05 22:51:02]

It seems weird that there are two entry points (createExternalTextureLayer and
createExternalTextureLayerForMailbox) both taking WebExternalTextureLayerClient.
Maybe WebExternalTextureLayerClient should be subclassed into e.g.
WebExternalTextureMailboxClient and the new entry points added to the subclass.

[alexst (slow to review), 2013/04/06 01:25:38]

Once mailboxes work across the board, the texture ID path should just go away.
My thought was after we flip the switch, we could just delete the old entry
points.

[Ken Russell (switch to Gerrit), 2013/04/08 18:22:56]

OK, that sounds fine.

+
     virtual WebImageLayer* createImageLayer() { return 0; }
 
     virtual WebSolidColorLayer* createSolidColorLayer() { return 0; }