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

Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
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 }
 
 AffineTransform HTMLCanvasElement::baseTransform() const
 {
     ASSERT(hasImageBuffer() && !m_didFailToCreateImageBuffer);
     return m_imageBuffer->baseTransform();
 }
 
 void HTMLCanvasElement::didChangeVisibilityState(PageVisibilityState visibility)
 {
-    if (hasImageBuffer()) {
-        bool hidden = visibility != PageVisibilityStateVisible;
-        if (hidden) {
-            clearCopiedImage();
-            if (is3D()) {
-                discardImageBuffer();
-            }
-        }
-        if (hasImageBuffer()) {
-            m_imageBuffer->setIsHidden(hidden);
+    if (!m_context)
+        return;
+    bool hidden = visibility != PageVisibilityStateVisible;
+    m_context->setIsHidden(hidden);
+    if (hidden) {
+        clearCopiedImage();
+        if (is3D()) {
+            discardImageBuffer();
         }
     }
 }
 
 void HTMLCanvasElement::didMoveToNewDocument(Document& oldDocument)
 {
     setObservedDocument(document());
     HTMLElement::didMoveToNewDocument(oldDocument);
 }
 
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 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }