WebGL: Don't destroy mailbox textures in the destructor until they're released.

Source/platform/graphics/gpu/DrawingBuffer.h

Index: Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.h b/Source/platform/graphics/gpu/DrawingBuffer.h
index b74f4cf2fbb0ebd5241016bf5986b8ea396a1b68..66c397021d0fc43fd4f700c0e5b29a53b9ff7456 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -72,6 +72,7 @@ class PLATFORM_EXPORT DrawingBuffer : public RefCounted<DrawingBuffer>, public b
         blink::WebExternalTextureMailbox mailbox;
         unsigned textureId;
         IntSize size;
+        RefPtr<DrawingBuffer> m_parentDrawingBuffer;

[Ken Russell (switch to Gerrit), 2014/04/15 02:24:02]

Please document the semantics.

"This keeps the parent drawing buffer alive as long as the compositor is
referring to one of the mailboxes it produced. The parent drawing buffer is
cleared when the MailboxInfo is added to the m_recycledMailboxes vector, and
also when the MailboxInfo is released while destruction is in progress."

     };
 public:
     enum PreserveDrawingBuffer {
@@ -83,6 +84,9 @@ public:
 
     virtual ~DrawingBuffer();
 
+    // Destruction will be completed after all mailboxes are received.

[Ken Russell (switch to Gerrit), 2014/04/15 02:24:02]

received -> released

+    void beginDestruction();
+
     // Issues a glClear() on all framebuffers associated with this DrawingBuffer. The caller is responsible for
     // making the context current and setting the clear values and masks. Modifies the framebuffer binding.
     void clearFramebuffers(GLbitfield clearMask);
@@ -138,12 +142,14 @@ public:
     void paintRenderingResultsToCanvas(ImageBuffer*);
     PassRefPtr<Uint8ClampedArray> paintRenderingResultsToImageData(int&, int&);
 
-private:
+protected: // For unittests
     DrawingBuffer(PassOwnPtr<blink::WebGraphicsContext3D>, bool multisampleExtensionSupported,
                   bool packedDepthStencilExtensionSupported, PreserveDrawingBuffer, PassRefPtr<ContextEvictionManager>);
 
     bool initialize(const IntSize&);
-    void releaseResources();
+
+private:
+    void mailboxReleasedInDestructionInProgress(const blink::WebExternalTextureMailbox&);

[Ken Russell (switch to Gerrit), 2014/04/15 02:24:02]

Please rename this to "mailboxReleasedWhileDestructionInProgress" to improve the
grammar.

 
     unsigned createColorTexture(const IntSize& size = IntSize());
     // Create the depth/stencil and multisample buffers, if needed.
@@ -237,6 +243,7 @@ private:
     int m_maxTextureSize;
     int m_sampleCount;
     int m_packAlignment;
+    bool m_destructionInProgress;
 
     OwnPtr<blink::WebExternalTextureLayer> m_layer;