bluetooth: Detect and fix incorrect thread usage of BluetoothDispatcherHost.

content/browser/bluetooth/bluetooth_dispatcher_host.h

Index: content/browser/bluetooth/bluetooth_dispatcher_host.h
diff --git a/content/browser/bluetooth/bluetooth_dispatcher_host.h b/content/browser/bluetooth/bluetooth_dispatcher_host.h
index cc6cd9d5cb21ad43b4cfe9e008f373a6053b5fd7..8a8714f17bf49c1a84d065a7ba5dfe001ec64d03 100644
--- a/content/browser/bluetooth/bluetooth_dispatcher_host.h
+++ b/content/browser/bluetooth/bluetooth_dispatcher_host.h
@@ -187,7 +187,10 @@ class CONTENT_EXPORT BluetoothDispatcherHost final
   // 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);