Block http auth prompt for cross-origin image subresources by default.

Still allow this to be bypassed with a pref for those really rare corner
cases where images are loaded cross-origin by design and the session
hasn't been/can't be authenticated ahead of time.

netwerk/protocol/http/nsHttpChannelAuthProvider.h

@@ -179,10 +179,11 @@ private:
 
     RefPtr<nsHttpHandler>           mHttpHandler;  // keep gHttpHandler alive
 
-    // A variable holding the preference settings to whether to open HTTP
+    // Variables holding the preference settings for whether to open HTTP
     // authentication credentials dialogs for sub-resources and cross-origin
     // sub-resources.
     static uint32_t                   sAuthAllowPref;
+    static bool                       sImgCrossOriginAuthAllowPref;
     nsCOMPtr<nsICancelable>           mGenerateCredentialsCancelable;
 };