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..7ab6ce30f5b2908f8ace55941194270882143c46 100644
--- a/content/browser/bluetooth/bluetooth_dispatcher_host.h
+++ b/content/browser/bluetooth/bluetooth_dispatcher_host.h
@@ -187,6 +187,10 @@ class CONTENT_EXPORT BluetoothDispatcherHost final
   // sessions when other sessions are active.
   base::Timer discovery_session_timer_;
 
+  // At least one WeakPtr must always exist to enforce all factoried WeakPtr
+  // objects are bound to UI thread.
+  base::WeakPtr<BluetoothDispatcherHost> weak_ptr_on_ui_thread_;
+
   // Must be last member, see base/memory/weak_ptr.h documentation
   base::WeakPtrFactory<BluetoothDispatcherHost> weak_ptr_factory_;