[Chromecast] Add mock implementation of CastMetricsHelper.

chromecast/base/metrics/mock_cast_metrics_helper.cc

Index: chromecast/base/metrics/mock_cast_metrics_helper.cc
diff --git a/chromecast/base/metrics/mock_cast_metrics_helper.cc b/chromecast/base/metrics/mock_cast_metrics_helper.cc
new file mode 100644
index 0000000000000000000000000000000000000000..2af76e04897a8cfd41b1ae67609d4be7d5800a2b
--- /dev/null
+++ b/chromecast/base/metrics/mock_cast_metrics_helper.cc
@@ -0,0 +1,15 @@
+// Copyright 2016 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.
+
+#include "chromecast/base/metrics/mock_cast_metrics_helper.h"
+
+namespace chromecast {
+namespace metrics {
+
+MockCastMetricsHelper::MockCastMetricsHelper() = default;
+
+MockCastMetricsHelper::~MockCastMetricsHelper() = default;
+
+}  // namespace metrics
+}  // namespace chromecast