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 b8a51abb68908f13cc491b4e8e54be221b18badb..60e973a3fc0806404ae2372d0a4134de57f8ee02 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -129,6 +129,7 @@ public:
     void markContentsChanged();
     void markLayerComposited();
     bool layerComposited() const;
+    void setIsHidden(bool);
 
     blink::WebLayer* platformLayer();
     void paintCompositedResultsToCanvas(ImageBuffer*);
@@ -158,7 +159,7 @@ protected: // For unittests
     bool initialize(const IntSize&);
 
 private:
-    void mailboxReleasedWhileDestructionInProgress(const blink::WebExternalTextureMailbox&);
+    void mailboxReleasedWithoutRecycling(const blink::WebExternalTextureMailbox&);
 
     unsigned createColorTexture(const IntSize& size = IntSize());
     // Create the depth/stencil and multisample buffers, if needed.
@@ -175,6 +176,7 @@ private:
     PassRefPtr<MailboxInfo> recycledMailbox();
     PassRefPtr<MailboxInfo> createNewMailbox(unsigned);
     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);
@@ -255,6 +257,7 @@ private:
     int m_sampleCount;
     int m_packAlignment;
     bool m_destructionInProgress;
+    bool m_isHidden;
 
     OwnPtr<blink::WebExternalTextureLayer> m_layer;