Reuse WeakFactory from U2fDevice derived classes

device/u2f/u2f_device.h

Index: device/u2f/u2f_device.h
diff --git a/device/u2f/u2f_device.h b/device/u2f/u2f_device.h
index 77c8133ff5afd7d613516366695dca1a9fdfce04..9537d44bac422e5c3d795ebee7461cc6b788028b 100644
--- a/device/u2f/u2f_device.h
+++ b/device/u2f/u2f_device.h
@@ -60,6 +60,7 @@ class U2fDevice {
   // the device communication transaction.
   virtual void DeviceTransact(std::unique_ptr<U2fApduCommand> command,
                               const DeviceCallback& callback) = 0;
+  virtual base::WeakPtr<U2fDevice> GetWeakPtr() = 0;
 
   uint32_t channel_id_;
   uint8_t capabilities_;
@@ -82,8 +83,6 @@ class U2fDevice {
               bool success,
               std::unique_ptr<U2fApduResponse> response);
 
-  base::WeakPtrFactory<U2fDevice> weak_factory_;
-
   DISALLOW_COPY_AND_ASSIGN(U2fDevice);
 };