Fixed crash in accessibility code in LegacyRenderWidgetHost.

R=dougt@chromium.org, dmazzoni@chromium.org, ananta@chromium.org
BUG=703369
TESTED=Manually with Jaws 18

Review-Url: https://codereview.chromium.org/2879623003
Cr-Commit-Position: refs/heads/master@{#471191}

content/browser/renderer_host/legacy_render_widget_host_win.cc

@@ -182,7 +182,7 @@ LRESULT LegacyRenderWidgetHostHWND::OnGetObject(UINT message,
   BrowserAccessibilityManagerWin* manager =
       static_cast<BrowserAccessibilityManagerWin*>(
           rwhi->GetRootBrowserAccessibilityManager());
-  if (!manager)
+  if (!manager || !manager->GetRoot())
     return static_cast<LRESULT>(0L);
 
   base::win::ScopedComPtr<IAccessible> root(