CHECK that the frames that UserScriptScheduler gets are non-NULL. We're seeing

chrome/renderer/extensions/user_script_scheduler.cc

Index: chrome/renderer/extensions/user_script_scheduler.cc
diff --git a/chrome/renderer/extensions/user_script_scheduler.cc b/chrome/renderer/extensions/user_script_scheduler.cc
index d4b77d1aa96a50e98bd099763cd3e79296ef9792..9b346f7b23f8d75d342d9cdd2c96fe67c8ef9bf7 100644
--- a/chrome/renderer/extensions/user_script_scheduler.cc
+++ b/chrome/renderer/extensions/user_script_scheduler.cc
@@ -171,9 +171,13 @@ void UserScriptScheduler::ExecuteCodeImpl(
   if (params.user_gesture)
     gesture.reset(new blink::WebScopedUserGesture);
 
+  GURL top_url = frame_->document().url();
+
   for (std::vector<WebFrame*>::iterator frame_it = frame_vector.begin();
        frame_it != frame_vector.end(); ++frame_it) {
     WebFrame* child_frame = *frame_it;
+    CHECK(child_frame) << top_url;
+
     // We recheck access here in the renderer for extra safety against races
     // with navigation.
     //
@@ -187,7 +191,7 @@ void UserScriptScheduler::ExecuteCodeImpl(
     if (!params.is_web_view &&
         !PermissionsData::CanExecuteScriptOnPage(extension,
                                                  child_frame->document().url(),
-                                                 frame_->document().url(),
+                                                 top_url,
                                                  extension_helper->tab_id(),
                                                  NULL,
                                                  -1,