Oilpan: keep ImageData on the heap by default.

Source/core/html/canvas/CanvasRenderingContext2DTest.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 
 #include "core/html/canvas/CanvasRenderingContext2D.h"
 
 #include "core/frame/FrameView.h"
 #include "core/frame/ImageBitmap.h"
@@ skipping 71 matching lines @@
 
     void createContext(OpacityMode);
 
 private:
     OwnPtr<DummyPageHolder> m_dummyPageHolder;
     RefPtrWillBePersistent<HTMLDocument> m_document;
     RefPtrWillBePersistent<HTMLCanvasElement> m_canvasElement;
 
 protected:
     // Pre-canned objects for testing
-    RefPtrWillBePersistent<ImageData> m_fullImageData;
-    RefPtrWillBePersistent<ImageData> m_partialImageData;
+    Persistent<ImageData> m_fullImageData;
+    Persistent<ImageData> m_partialImageData;
     FakeImageSource m_opaqueBitmap;
     FakeImageSource m_alphaBitmap;
     StringOrCanvasGradientOrCanvasPattern m_opaqueGradient;
     StringOrCanvasGradientOrCanvasPattern m_alphaGradient;
 };
 
 CanvasRenderingContext2DTest::CanvasRenderingContext2DTest()
     : m_opaqueBitmap(IntSize(10, 10), OpaqueBitmap)
     , m_alphaBitmap(IntSize(10, 10), TransparentBitmap)
 { }
@@ skipping 494 matching lines @@
     // The canvasChanged notification must be immediate, and not deferred until paint time
     // because offscreen canvases, which are not painted, also need to emit notifications.
     EXPECT_CALL(*observer, canvasChanged(&canvasElement(), FloatRect(0, 0, 1, 1))).Times(1);
     context2d()->fillRect(0, 0, 1, 1);
     Mock::VerifyAndClearExpectations(observer.get());
 
     canvasElement().removeObserver(observer.get());
 }
 
 } // unnamed namespace