WebGL: Free temporary GPU resources held by inactive or hidden WebGL.

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 4c457b7c7b747dc97b8763a629758a6bbe6de54f..545918e95e4265cdca195252de0871abb62f12ba 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -142,6 +142,7 @@ public:
     void markContentsChanged();
     void markLayerComposited();
     bool layerComposited() const;
+    void setIsHidden(bool);
 
     blink::WebLayer* platformLayer();
     void paintCompositedResultsToCanvas(ImageBuffer*);
@@ -178,7 +179,7 @@ protected: // For unittests
     bool initialize(const IntSize&);
 
 private:
-    void mailboxReleasedWhileDestructionInProgress(const blink::WebExternalTextureMailbox&);
+    void mailboxReleasedWithoutRecycling(const blink::WebExternalTextureMailbox&);
 
     unsigned createColorTexture();
     // Create the depth/stencil and multisample buffers, if needed.
@@ -195,6 +196,7 @@ private:
     PassRefPtr<MailboxInfo> recycledMailbox();
     PassRefPtr<MailboxInfo> createNewMailbox(const TextureInfo&);
     void deleteMailbox(const blink::WebExternalTextureMailbox&);
+    void freeRecycledMailboxes();
 
     // Updates the current size of the buffer, ensuring that s_currentResourceUsePixels is updated.
     void setSize(const IntSize& size);
@@ -279,6 +281,7 @@ private:
     int m_sampleCount;
     int m_packAlignment;
     bool m_destructionInProgress;
+    bool m_isHidden;
 
     OwnPtr<blink::WebExternalTextureLayer> m_layer;