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..9f8c187133b50d6afbfc19efb9c334b245ee1e8e 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, Canvas2DLayerBridgePtr* outLayerBridge, OpacityMode opacityMode)
 {
     RefPtr<GraphicsContext3D> context3D = SharedGraphicsContext3D::get();
     if (!context3D)
@@ -70,7 +70,7 @@ static SkCanvas* createAcceleratedCanvas(const IntSize& size, OwnPtr<Canvas2DLay
     *outLayerBridge = Canvas2DLayerBridge::create(context3D.release(), size, bridgeOpacityMode);
     // If canvas buffer allocation failed, debug build will have asserted
     // For release builds, we must verify whether the device has a render target
-    return (*outLayerBridge) ? (*outLayerBridge)->getCanvas() : 0;
+    return outLayerBridge->get() ? (*outLayerBridge)->getCanvas() : 0;
 }
 
 static SkCanvas* createNonPlatformCanvas(const IntSize& size)
@@ -163,7 +163,7 @@ ImageBuffer::~ImageBuffer()
 
 GraphicsContext* ImageBuffer::context() const
 {
-    if (m_layerBridge) {
+    if (m_layerBridge.get()) {
         // We're using context acquisition as a signal that someone is about to render into our buffer and we need
         // to be ready. This isn't logically const-correct, hence the cast.
         const_cast<Canvas2DLayerBridge*>(m_layerBridge.get())->contextAcquired();
@@ -205,12 +205,12 @@ BackingStoreCopy ImageBuffer::fastCopyImageMode()
 
 WebKit::WebLayer* ImageBuffer::platformLayer() const
 {
-    return m_layerBridge ? m_layerBridge->layer() : 0;
+    return m_layerBridge.get() ? m_layerBridge->layer() : 0;
 }
 
 bool ImageBuffer::copyToPlatformTexture(GraphicsContext3D& context, Platform3DObject texture, GC3Denum internalFormat, GC3Denum destType, GC3Dint level, bool premultiplyAlpha, bool flipY)
 {
-    if (!m_layerBridge || !platformLayer() || !isValid())
+    if (!m_layerBridge.get() || !platformLayer() || !isValid())
         return false;
 
     Platform3DObject sourceTexture = m_layerBridge->backBufferTexture();