Reuse WeakFactory from U2fDevice derived classes

device/u2f/mock_u2f_device.cc

Index: device/u2f/mock_u2f_device.cc
diff --git a/device/u2f/mock_u2f_device.cc b/device/u2f/mock_u2f_device.cc
index 4f8bdc46f0e691c9c5febcdbde713c7c4d9dbd61..e833bbf6ef886891fbe0fe4d5c08f2695d78210a 100644
--- a/device/u2f/mock_u2f_device.cc
+++ b/device/u2f/mock_u2f_device.cc
@@ -6,7 +6,7 @@
 
 namespace device {
 
-MockU2fDevice::MockU2fDevice() {}
+MockU2fDevice::MockU2fDevice() : weak_factory_(this) {}
 
 MockU2fDevice::~MockU2fDevice() {}
 
@@ -50,4 +50,8 @@ void MockU2fDevice::WinkDoNothing(const WinkCallback& cb) {
   cb.Run();
 }
 
+base::WeakPtr<U2fDevice> MockU2fDevice::GetWeakPtr() {
+  return weak_factory_.GetWeakPtr();
+}
+
 }  // namespace device