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

Source/core/imagebitmap/ImageBitmapFactories.cpp

 /*
  * Copyright (c) 2013, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 160 matching lines @@
     if (!canvas->originClean()) {
         exceptionState.throwSecurityError("The canvas element provided is tainted with cross-origin data.");
         return ScriptPromise();
     }
     if (!sw || !sh) {
         exceptionState.throwDOMException(IndexSizeError, String::format("The source %s provided is 0.", sw ? "height" : "width"));
         return ScriptPromise();
     }
 
     // FIXME: make ImageBitmap creation asynchronous crbug.com/258082
-    return fulfillImageBitmap(scriptState, canvas->buffer() ? ImageBitmap::create(canvas, IntRect(sx, sy, sw, sh)) : nullptr);
+    return fulfillImageBitmap(scriptState, canvas->isPaintable() ? ImageBitmap::create(canvas, IntRect(sx, sy, sw, sh)) : nullptr);
 }
 
 ScriptPromise ImageBitmapFactories::createImageBitmap(ScriptState* scriptState, EventTarget& eventTarget, Blob* blob, ExceptionState& exceptionState)
 {
     ImageBitmapLoader* loader = ImageBitmapFactories::ImageBitmapLoader::create(from(eventTarget), IntRect(), scriptState);
     ScriptPromise promise = loader->promise();
     from(eventTarget).addLoader(loader);
     loader->loadBlobAsync(eventTarget.executionContext(), blob);
     return promise;
 }
@@ skipping 138 matching lines @@
 {
     rejectPromise();
 }
 
 void ImageBitmapFactories::ImageBitmapLoader::trace(Visitor* visitor)
 {
     visitor->trace(m_factory);
 }
 
 } // namespace blink