canvas: make a temporary buffer when a context doesn't exist.

Source/core/imagebitmap/ImageBitmapFactories.cpp

Index: Source/core/imagebitmap/ImageBitmapFactories.cpp
diff --git a/Source/core/imagebitmap/ImageBitmapFactories.cpp b/Source/core/imagebitmap/ImageBitmapFactories.cpp
index ac39e7032e001e5c89048930235a59cff6d70ac8..9dd9d6a1ee11870129e87d99cab288f7502cde72 100644
--- a/Source/core/imagebitmap/ImageBitmapFactories.cpp
+++ b/Source/core/imagebitmap/ImageBitmapFactories.cpp
@@ -178,7 +178,8 @@ ScriptPromise ImageBitmapFactories::createImageBitmap(ScriptState* scriptState,
     }
 
     // FIXME: make ImageBitmap creation asynchronous crbug.com/258082
-    return fulfillImageBitmap(scriptState, canvas->buffer() ? ImageBitmap::create(canvas, IntRect(sx, sy, sw, sh)) : nullptr);
+    RefPtr<Image> canvasImage = canvas->copiedImage(BackBuffer);

[Justin Novosad, 2014/11/27 00:49:31]

This seems like a step in the wrong direction. We want to move toward making
this asynchronous (see comment above). Requesting a resolved Image here is the
opposite of that. Let me explain. If the canvas is stored as a display list or
use deferred rendering (via Canvas2DLayerBridge), the objective would be to
return immediately with an unfulfilled Promise object.

[dshwang, 2014/11/27 15:12:11]

Thank you for explaining. I rollback this change.

+    return fulfillImageBitmap(scriptState, canvasImage ? ImageBitmap::create(canvasImage.get(), IntRect(sx, sy, sw, sh)) : nullptr);
 }
 
 ScriptPromise ImageBitmapFactories::createImageBitmap(ScriptState* scriptState, EventTarget& eventTarget, Blob* blob, ExceptionState& exceptionState)