Change Canvas2DLayerBridge to stay alive until last mailbox is returned.

Source/core/platform/graphics/chromium/Canvas2DLayerBridge.h

Index: Source/core/platform/graphics/chromium/Canvas2DLayerBridge.h
diff --git a/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.h b/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.h
index 12f87fa0f0dd9af0e2c5a6eb8c314d46ec4bd97f..658a739e82102e7cef3baa1e35d7e4bf8b5ca048 100644
--- a/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.h
+++ b/Source/core/platform/graphics/chromium/Canvas2DLayerBridge.h
@@ -43,7 +43,7 @@ class WebGraphicsContext3D;
 
 namespace WebCore {
 
-class Canvas2DLayerBridge : public WebKit::WebExternalTextureLayerClient, public SkDeferredCanvas::NotificationClient, public DoublyLinkedListNode<Canvas2DLayerBridge> {
+class Canvas2DLayerBridge : public WebKit::WebExternalTextureLayerClient, public SkDeferredCanvas::NotificationClient, public DoublyLinkedListNode<Canvas2DLayerBridge>, public RefCounted<Canvas2DLayerBridge> {
     WTF_MAKE_NONCOPYABLE(Canvas2DLayerBridge);
 public:
     enum OpacityMode {
@@ -51,7 +51,7 @@ public:
         NonOpaque
     };
 
-    static PassOwnPtr<Canvas2DLayerBridge> create(PassRefPtr<GraphicsContext3D>, const IntSize&, OpacityMode);
+    static PassRefPtr<Canvas2DLayerBridge> create(PassRefPtr<GraphicsContext3D>, const IntSize&, OpacityMode);
 
     virtual ~Canvas2DLayerBridge();
 
@@ -81,9 +81,13 @@ public:
 
     bool isValid();
 
+    // Non-virtual overload of RefCounted::deref. RefPtr calls this one.
+    void deref();
+
 protected:
     Canvas2DLayerBridge(PassRefPtr<GraphicsContext3D>, SkDeferredCanvas*, OpacityMode);
     void setRateLimitingEnabled(bool);
+    bool isDead();
 
     SkDeferredCanvas* m_canvas;
     OwnPtr<WebKit::WebExternalTextureLayer> m_layer;
@@ -92,6 +96,8 @@ protected:
     bool m_didRecordDrawCommand;
     bool m_surfaceIsValid;
     int m_framesPending;
+    int m_liveMailboxCount;
+    bool m_destructionInProgress;
     bool m_rateLimitingEnabled;
 
     friend class WTF::DoublyLinkedListNode<Canvas2DLayerBridge>;
@@ -108,6 +114,7 @@ protected:
         WebKit::WebExternalTextureMailbox m_mailbox;
         SkAutoTUnref<SkImage> m_image;
         MailboxStatus m_status;
+        RefPtr<Canvas2DLayerBridge> m_parentLayerBridge;
 
         MailboxInfo(const MailboxInfo&);
         MailboxInfo() {}