Allow DrawingBuffer to present to a bitmap.

Source/core/platform/graphics/chromium/Canvas2DLayerBridge.cpp

Index: Source/core/platform/graphics/chromium/Canvas2DLayerBridge.cpp
diff --git a/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.cpp b/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.cpp
index 0c25d1d773f4465d4a9523222349b8654fd471a5..a0218e1d3ace8739e02471586330f756f39b88b3 100644
--- a/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.cpp
+++ b/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.cpp
@@ -169,9 +169,10 @@ WebGraphicsContext3D* Canvas2DLayerBridge::context()
     return m_context->webContext();
 }
 
-bool Canvas2DLayerBridge::prepareMailbox(WebKit::WebExternalTextureMailbox* outMailbox)
+bool Canvas2DLayerBridge::prepareMailbox(WebKit::WebExternalTextureMailbox* outMailbox, WebKit::WebExternalBitmap* bitmap)
 {
 #if ENABLE(CANVAS_USES_MAILBOX)
+    ASSERT(!bitmap);
     // Release to skia textures that were previouosly released by the
     // compositor. We do this before acquiring the next snapshot in
     // order to cap maximum gpu memory consumption.