Bug 1267075 - Part 3: cancel imgRequestProxy if asyncOpen2 failed. r=tnikkel

The problem is if we found a cache hit, then we could go through
ValidateRequestWithNewChannel to validate the cache.
Then if the CSP check fail(asyncOpen2() will fail), then the
imgRequestProxy will remain there, and cause the timeout.

I run into problem when running mochitest
browser/base/content/test/general/browser_aboutHome.js in non-e10s mode.
In the beginning, browser.xul will load defaultFavicon.png, will create
an image cache there.
Next time when the test starts to run, when it loads about:home, then it
will try to load defaultFavicon.png, it will found an image cache hit
(loaded previously by browser.xul), and call
ValidateRequestWithNewChannel there, however the asyncOpen2 call failed,
and the imgRequestProxy is added to the loadGroup of about:home, and
never be notified until timeout.

image/imgLoader.cpp

@@ -1747,9 +1747,9 @@ imgLoader::ValidateRequestWithNewChannel(imgRequest* request,
 
   mozilla::net::PredictorLearn(aURI, aInitialDocumentURI,
                                nsINetworkPredictor::LEARN_LOAD_SUBRESOURCE, aLoadGroup);
-
   rv = newChannel->AsyncOpen2(listener);
   if (NS_WARN_IF(NS_FAILED(rv))) {
+    req->CancelAndForgetObserver(rv);
     return false;
   }