NetworkService: Fix undefined behavior in RegisterNonNetworkSubresourceURLLoaderFactories.

In ChromeContentBrowserClient::RegisterNonNetworkSubresourceURLLoaderFactories,
it's possible that no ExtensionWebContentsObserver is attached to the
WebContents for the given RenderFrameHost. Return early in that case.

BUG=818875

Change-Id: Ifca9cc67d36fe5aee0da496952125ef8ee199849
Reviewed-on: https://chromium-review.googlesource.com/950134Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541014}

chrome/browser/chrome_content_browser_client.cc

@@ -3813,6 +3813,11 @@ void ChromeContentBrowserClient::
       extensions::ChromeExtensionWebContentsObserver::FromWebContents(
           web_contents);
 
+  // There is nothing to do if no ChromeExtensionWebContentsObserver is attached
+  // to the |web_contents|.
+  if (!web_observer)
+    return;
+
   const Extension* extension =
       web_observer->GetExtensionFromFrame(frame_host, false);
   if (!extension)