Bug 1139845 - Fix ChromeProcessController crash when there is no document element. r=botond

--HG--
extra : rebase_source : 108a0ed9e25d46ed77b6deefa081e1e7d19cffe9

gfx/layers/apz/util/ChromeProcessController.cpp

@@ -49,9 +49,13 @@ ChromeProcessController::InitializeRoot()
   if (!presShell) {
     return;
   }
+
   MOZ_ASSERT(presShell->GetDocument());
   nsIContent* content = presShell->GetDocument()->GetDocumentElement();
-  MOZ_ASSERT(content);
+  if (!content) {
+    return;
+  }
+
   uint32_t presShellId;
   FrameMetrics::ViewID viewId;
   if (APZCCallbackHelper::GetOrCreateScrollIdentifiers(content, &presShellId, &viewId)) {