Allow DrawingBuffer to present to a bitmap.

public/platform/WebExternalTextureLayerClient.h

Index: public/platform/WebExternalTextureLayerClient.h
diff --git a/public/platform/WebExternalTextureLayerClient.h b/public/platform/WebExternalTextureLayerClient.h
index 4829600c21f94fd14262fd417331b98c55995be2..584b9bb3215da0a8b9060a1c06b3a041e950b354 100644
--- a/public/platform/WebExternalTextureLayerClient.h
+++ b/public/platform/WebExternalTextureLayerClient.h
@@ -32,6 +32,7 @@
 namespace WebKit {
 
 class WebGraphicsContext3D;
+class WebExternalBitmap;
 struct WebExternalTextureMailbox;
 
 class WebTextureUpdater {
@@ -49,7 +50,7 @@ public:
 
     // Returns true and provides a mailbox if a new frame is available.
     // Returns false if no new data is available and the old mailbox is to be reused.
-    virtual bool prepareMailbox(WebExternalTextureMailbox*) = 0;
+    virtual bool prepareMailbox(WebExternalTextureMailbox*, WebExternalBitmap *output_bitmap = 0) = 0;

[jamesr, 2013/07/03 21:53:02]

* goes with the type, not the parameter name. this parameter name isn't too
useful so blink style says you should omit it.

please update the comments for prepareMailbox() to say what happens if the
second parameter isn't null

 
     // Notifies the client when a mailbox is no longer in use by the compositor and provides
     // a sync point to wait on before the mailbox could be consumes again by the client.