webRequest: Check weak ptr before dispatching event

BUG=581148

Review URL: https://codereview.chromium.org/1632963002

Cr-Commit-Position: refs/heads/master@{#371418}

extensions/browser/api/web_request/web_request_api.cc

@@ -1072,6 +1072,9 @@ void ExtensionWebRequestEventRouter::DispatchEventToListeners(
         continue;
     }
 
+    if (!listener->ipc_sender.get())
+      continue;
+
     // Filter out the optional keys that this listener didn't request.
     scoped_ptr<base::ListValue> args_filtered(new base::ListValue);
     args_filtered->Append(