Implement origin-clean algorithm for ImageBitmap.

This resolves #973.

dom/html/HTMLCanvasElement.cpp

@@ -1000,7 +1000,7 @@ HTMLCanvasElement::GetSize()
 }
 
 bool
-HTMLCanvasElement::IsWriteOnly()
+HTMLCanvasElement::IsWriteOnly() const
 {
   return mWriteOnly;
 }

dom/html/HTMLCanvasElement.h

@@ -224,9 +224,9 @@ public:
   nsIntSize GetSize();
 
   /**
-   * Determine whether the canvas is write-only.
+   * Determine whether the canvas is write-only (tainted).
    */
-  bool IsWriteOnly();
+  bool IsWriteOnly() const;
 
   /**
    * Force the canvas to be write-only.