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

Source/core/html/HTMLCanvasElement.h

Index: Source/core/html/HTMLCanvasElement.h
diff --git a/Source/core/html/HTMLCanvasElement.h b/Source/core/html/HTMLCanvasElement.h
index 7244986e7c1536d6e8e8298932dbe871078b4a54..44fe971c17ceec571208407b4a7a4549e761261d 100644
--- a/Source/core/html/HTMLCanvasElement.h
+++ b/Source/core/html/HTMLCanvasElement.h
@@ -31,6 +31,7 @@
 #include "core/html/HTMLElement.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/IntSize.h"
+#include "platform/graphics/Canvas2DLayerBridge.h"
 #include "wtf/Forward.h"
 
 #define DefaultInterpolationQuality InterpolationMedium
@@ -58,7 +59,7 @@ public:
     virtual void canvasDestroyed(HTMLCanvasElement*) = 0;
 };
 
-class HTMLCanvasElement FINAL : public HTMLElement {
+class HTMLCanvasElement FINAL : public HTMLElement, public DocumentVisibilityObserver {
 public:
     static PassRefPtr<HTMLCanvasElement> create(Document&);
     virtual ~HTMLCanvasElement();
@@ -127,6 +128,12 @@ public:
 
     virtual InsertionNotificationRequest insertedInto(ContainerNode*) OVERRIDE;
 
+    // DocumentVisibilityObserver implementation
+    virtual void didChangeVisibilityState(PageVisibilityState) OVERRIDE;
+
+protected:
+    virtual void didMoveToNewDocument(Document& oldDocument) OVERRIDE;
+
 private:
     explicit HTMLCanvasElement(Document&);