Bug 1045096. Make sure initial about:blank in iframes have a nsDOMNavigationTiming object, so we don't end up with window.performance throwing when accessed on their window. r=smaug

docshell/base/nsDocShell.cpp

@@ -7366,15 +7366,18 @@ nsDocShell::CreateAboutBlankContentViewer(nsIPrincipal* aPrincipal,
   // mContentViewer->PermitUnload may release |this| docshell.
   nsCOMPtr<nsIDocShell> kungFuDeathGrip(this);
   
+  // Make sure timing is created.  But first record whether we had it
+  // already, so we don't clobber the timing for an in-progress load.
+  bool hadTiming = mTiming;
+  MaybeInitTiming();
   if (mContentViewer) {
     // We've got a content viewer already. Make sure the user
     // permits us to discard the current document and replace it
     // with about:blank. And also ensure we fire the unload events
     // in the current document.
 
-    // Make sure timing is created. Unload gets fired first for
+    // Unload gets fired first for
     // document loaded from the session history.
-    MaybeInitTiming();
     mTiming->NotifyBeforeUnload();
 
     bool okToUnload;
@@ -7459,6 +7462,12 @@ nsDocShell::CreateAboutBlankContentViewer(nsIPrincipal* aPrincipal,
   // The transient about:blank viewer doesn't have a session history entry.
   SetHistoryEntry(&mOSHE, nullptr);
 
+  // Clear out our mTiming like we would in EndPageLoad, if we didn't
+  // have one before entering this function.
+  if (!hadTiming) {
+    mTiming = nullptr;
+  }
+
   return rv;
 }