Fix crash in device_forwarder

tools/android/forwarder2/device_forwarder_main.cc

Index: tools/android/forwarder2/device_forwarder_main.cc
diff --git a/tools/android/forwarder2/device_forwarder_main.cc b/tools/android/forwarder2/device_forwarder_main.cc
index fdf5fe54a54a0a8fd9453cdb6af0e349ab91ebbe..09790f98b5f90fc5bea5dbbd6280ba2aca233cd2 100644
--- a/tools/android/forwarder2/device_forwarder_main.cc
+++ b/tools/android/forwarder2/device_forwarder_main.cc
@@ -52,19 +52,9 @@ class ServerDelegate : public Daemon::ServerDelegate {
     if (!controller_thread_.get())
       return;
     // The DeviceController instance, if any, is constructed on the controller
-    // thread. Make sure that it gets deleted on that same thread. Note that
-    // DeleteSoon() is not used here since it would imply reading |controller_|

[jbudorick, 2016/01/08 21:41:08]

I hadn't reviewed yet because I'd like to look into this a bit more. Switching
to DeleteSoon despite the previous comment is a little concerning.

[Xianzhu, 2016/01/08 21:47:03]

I don't think the previous comment is correct. The only risk of reading
controller_ from the main thread is that ~ServerDelegate() is called at the same
time of StartController(), but the original code could not avoid the risk. The
risk is very low so I would just ignore it.

[jbudorick, 2016/01/08 22:11:37]

It looks like the original would avoid that risk by posting StartController to
controller_thread_.

I'm not crazy about this, but it does seem to be an improvement over the
existing code. Could we add something like a WaitableEvent that gets set by
DeleteControllerOnInternalThread after resetting controller_ that this dtor
could wait on after posting the task?

-    // from the main thread while it's set on the internal thread.
-    controller_thread_->task_runner()->PostTask(
-        FROM_HERE,
-        base::Bind(&ServerDelegate::DeleteControllerOnInternalThread,
-                   base::Unretained(this)));
-  }
-
-  void DeleteControllerOnInternalThread() {
-    DCHECK(
-        controller_thread_->task_runner()->RunsTasksOnCurrentThread());
-    controller_.reset();
+    // thread. Make sure that it gets deleted on that same thread.
+    controller_thread_->task_runner()->DeleteSoon(
+        FROM_HERE, controller_.release());
   }
 
   // Daemon::ServerDelegate: