Free temporary GPU and memory resources held by inactive or hidden 2D canvases

Source/platform/graphics/ImageBufferSurface.h

Index: Source/platform/graphics/ImageBufferSurface.h
diff --git a/Source/platform/graphics/ImageBufferSurface.h b/Source/platform/graphics/ImageBufferSurface.h
index 60f955284ba79eb743a2f83814604939c97a6bc6..f05c021ff0b368eca0b8a6fb7dd85340c65788ab 100644
--- a/Source/platform/graphics/ImageBufferSurface.h
+++ b/Source/platform/graphics/ImageBufferSurface.h
@@ -65,6 +65,7 @@ public:
     virtual const SkBitmap& cachedBitmap() const;
     virtual void invalidateCachedBitmap() { }
     virtual void updateCachedBitmapIfNeeded() { }
+    virtual void setIsHidden(bool) { }
 
     OpacityMode opacityMode() const { return m_opacityMode; }
     const IntSize& size() const { return m_size; }
@@ -75,7 +76,9 @@ protected:
     ImageBufferSurface(const IntSize& size, OpacityMode opacityMode)
         : m_opacityMode(opacityMode)
         , m_size(size)
-    { }
+    {
+        setIsHidden(false);
+    }
 
 private:
     OpacityMode m_opacityMode;