Change Canvas2DLayerBridge to stay alive until last mailbox is returned.

Source/core/platform/graphics/ImageBuffer.cpp

Index: Source/core/platform/graphics/ImageBuffer.cpp
diff --git a/Source/core/platform/graphics/ImageBuffer.cpp b/Source/core/platform/graphics/ImageBuffer.cpp
index 48f2f54951c38fc7cdced7bee47c1be5477d7df3..25200f044fc849e6e8f199a9f86aeb9ef49fc7a2 100644
--- a/Source/core/platform/graphics/ImageBuffer.cpp
+++ b/Source/core/platform/graphics/ImageBuffer.cpp
@@ -61,7 +61,7 @@ using namespace std;
 
 namespace WebCore {
 
-static SkCanvas* createAcceleratedCanvas(const IntSize& size, OwnPtr<Canvas2DLayerBridge>* outLayerBridge, OpacityMode opacityMode)
+static SkCanvas* createAcceleratedCanvas(const IntSize& size, RefPtr<Canvas2DLayerBridge>* outLayerBridge, OpacityMode opacityMode)
 {
     RefPtr<GraphicsContext3D> context3D = SharedGraphicsContext3D::get();
     if (!context3D)
@@ -174,7 +174,7 @@ GraphicsContext* ImageBuffer::context() const
 
 bool ImageBuffer::isValid() const
 {
-    if (m_layerBridge.get())
+    if (m_layerBridge)
         return const_cast<Canvas2DLayerBridge*>(m_layerBridge.get())->isValid();
     return true;
 }