Bug 1479588 - Check mOwnerContent when starting new remote browser, r=bzbarsky

Differential Revision: https://phabricator.services.mozilla.com/D2524

dom/base/nsFrameLoader.cpp

@@ -2562,6 +2562,10 @@ nsFrameLoader::TryRemoteBrowser()
 {
   NS_ASSERTION(!mRemoteBrowser, "TryRemoteBrowser called with a remote browser already?");
 
+  if (!mOwnerContent) {
+    return false;
+  }
+
   //XXXsmaug Per spec (2014/08/21) frameloader should not work in case the
   //         element isn't in document, only in shadow dom, but that will change
   //         https://www.w3.org/Bugs/Public/show_bug.cgi?id=26365#c0