Fix a race where backgroud page might ack an event before we're ready

    to receive the ack on browser/.

We perform a sanity check upon background page ACKing an event,
    by checking whether the ACKed message is expected to
    be ACKed. We store the pending ACK message IDs into a set
    |unacked_messages_|, but due to PostTask call to
    IncrementInFlightEventsOnUI, the event ack might arrive earlier
    than storing the pending message's ID. This results in incorectly
    kill the renderer process.

BUG=485440, 473821
Test=Rise Vision chrome app shouldn't crash anymore.

Review URL: https://codereview.chromium.org/1138643010

Cr-Commit-Position: refs/heads/master@{#330301}

extensions/browser/event_router.cc

@@ -155,14 +155,21 @@ void EventRouter::DispatchEvent(IPC::Sender* ipc_sender,
                                 const EventFilteringInfo& info) {
   int event_id = g_extension_event_id.GetNext();
 
+  if (!BrowserThread::CurrentlyOn(BrowserThread::UI)) {
+    // This is called from WebRequest API.
+    // TODO(lazyboy): Skip this entirely: http://crbug.com/488747.
+    BrowserThread::PostTask(
+        BrowserThread::UI, FROM_HERE,
+        base::Bind(&EventRouter::IncrementInFlightEventsOnUI,
+                   browser_context_id, extension_id, event_id, event_name));
+  } else {
+    IncrementInFlightEventsOnUI(browser_context_id, extension_id, event_id,
+                                event_name);
+  }
+
   DispatchExtensionMessage(ipc_sender, browser_context_id, extension_id,
                            event_id, event_name, event_args.get(), user_gesture,
                            info);
-
-  BrowserThread::PostTask(
-      BrowserThread::UI, FROM_HERE,
-      base::Bind(&EventRouter::IncrementInFlightEventsOnUI, browser_context_id,
-                 extension_id, event_id, event_name));
 }
 
 EventRouter::EventRouter(BrowserContext* browser_context,