forwarder2: Make the Forwarder instances operate on a single thread.

tools/android/forwarder2/device_listener.h

Index: tools/android/forwarder2/device_listener.h
diff --git a/tools/android/forwarder2/device_listener.h b/tools/android/forwarder2/device_listener.h
index 1daf3f6275bfcc2240d0a34e90954b550a84e821..c7724f457dd0c76fa3378678baa8ee25b19bbd56 100644
--- a/tools/android/forwarder2/device_listener.h
+++ b/tools/android/forwarder2/device_listener.h
@@ -95,8 +95,6 @@ class DeviceListener {
   // Task runner used for deletion set at construction time (i.e. the object is
   // deleted on the same thread it is created on).
   scoped_refptr<base::SingleThreadTaskRunner> deletion_task_runner_;
-  // See host_controller.h for explanations about lifetime and threading
-  // interactions between this thread and the ForwardersManager below.
   base::Thread thread_;
   ForwardersManager forwarders_manager_;