Added IID() method to fix build bustage.

gfx/src/nsIDeviceContext.h

@@ -66,6 +66,8 @@ typedef NI_ColorSpace IL_ColorSpace;
 class nsIDeviceContext : public nsISupports
 {
 public:
+  static const nsIID& IID() { static nsIID iid = NS_IDEVICE_CONTEXT_IID; return iid; }
+
   /**
    * Initialize the device context from a widget
    * @param aWidget a native widget to initialize the device context from