bluetooth: Detect and fix incorrect thread usage of BluetoothDispatcherHost.

content/browser/bluetooth/bluetooth_dispatcher_host.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_BLUETOOTH_BLUETOOTH_DISPATCHER_HOST_H_
 #define CONTENT_BROWSER_BLUETOOTH_BLUETOOTH_DISPATCHER_HOST_H_
 
 #include "base/basictypes.h"
 #include "base/id_map.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 169 matching lines @@
   // A BluetoothAdapter instance representing an adapter of the system.
   scoped_refptr<device::BluetoothAdapter> adapter_;
 
   // Automatically stops Bluetooth discovery a set amount of time after it was
   // started. We have a single timer for all of Web Bluetooth because it's
   // simpler than tracking timeouts for each RequestDeviceSession individually,
   // and because there's no harm in extending the length of a few discovery
   // sessions when other sessions are active.
   base::Timer discovery_session_timer_;
 
-  // Must be last member, see base/memory/weak_ptr.h documentation
+  // |weak_ptr_on_ui_thread_| provides weak pointers, e.g. for callbacks, and
+  // because it exists and has been bound to the UI thread enforces that all
+  // copies verify they are also used on the UI thread.
+  base::WeakPtr<BluetoothDispatcherHost> weak_ptr_on_ui_thread_;
   base::WeakPtrFactory<BluetoothDispatcherHost> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(BluetoothDispatcherHost);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_BLUETOOTH_BLUETOOTH_DISPATCHER_HOST_H_