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 5e87788df1bbdcc6f12c99e83879b7eab90bdc32..45f19ef7fcf95db5b585d398bced3226692fc63f 100644
--- a/Source/core/platform/graphics/ImageBuffer.cpp
+++ b/Source/core/platform/graphics/ImageBuffer.cpp
@@ -62,7 +62,7 @@ using namespace std;
 
 namespace WebCore {
 
-static SkCanvas* createAcceleratedCanvas(const IntSize& size, OwnPtr<Canvas2DLayerBridge>* outLayerBridge, OpacityMode opacityMode)
+static SkCanvas* createAcceleratedCanvas(const IntSize& size, Canvas2DLayerBridgePtr* outLayerBridge, OpacityMode opacityMode)
 {
     RefPtr<GraphicsContext3D> context3D = SharedGraphicsContext3D::get();
     if (!context3D)
@@ -175,7 +175,7 @@ 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;
 }