Trigger hardware accelerated canvas GC more aggressively.

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.
@@ skipping 676 matching lines @@
     visitor->trace(m_observers);
     visitor->trace(m_context);
 #endif
     DocumentVisibilityObserver::trace(visitor);
     HTMLElement::trace(visitor);
 }
 
 void HTMLCanvasElement::updateExternallyAllocatedMemory() const
 {
     int bufferCount = 0;
-    if (m_imageBuffer)
+    if (m_imageBuffer) {
         bufferCount++;
+        if (m_imageBuffer->isAccelerated()) {
+            // The number of internal GPU buffers vary between one (stable
+            // non-displayed state) and three (triple-buffered animations).
+            // Adding 2 is a pessimistic but relevant estimate.
+            // Note: These buffers might be allocated in GPU memory.
+            bufferCount += 2;
+        }
+    }
     if (m_copiedImage)
         bufferCount++;
 
+    // Four bytes per pixel per buffer.
     Checked<intptr_t, RecordOverflow> checkedExternallyAllocatedMemory = 4 * bufferCount;
     if (is3D())
         checkedExternallyAllocatedMemory += toWebGLRenderingContext(m_context.get())->externallyAllocatedBytesPerPixel();
 
     checkedExternallyAllocatedMemory *= width();
     checkedExternallyAllocatedMemory *= height();
     intptr_t externallyAllocatedMemory;
     if (checkedExternallyAllocatedMemory.safeGet(externallyAllocatedMemory) == CheckedState::DidOverflow)
         externallyAllocatedMemory = std::numeric_limits<intptr_t>::max();
 
@@ skipping 153 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }