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..86e59216679bee06807e7b0723d23daf620c7fd3 100644
--- a/public/platform/WebExternalTextureLayerClient.h
+++ b/public/platform/WebExternalTextureLayerClient.h
@@ -49,7 +49,11 @@ 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;
+    // If bitmap is true the data should be put in a texture in the mailbox.
+    virtual bool prepareMailbox(WebExternalTextureMailbox*, bool bitmap) = 0;
+
+    // Obsolete.
+    virtual bool prepareMailbox(WebExternalTextureMailbox* mailbox) { return prepareMailbox(mailbox, false); }
 
     // 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.