[Chromecast] Add mock implementation of CastMetricsHelper.

chromecast/base/metrics/BUILD.gn

 # Copyright 2015 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.
 
 source_set("metrics") {
   sources = [
     "cast_histograms.h",
     "cast_metrics_helper.cc",
     "cast_metrics_helper.h",
     "grouped_histogram.cc",
     "grouped_histogram.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
 source_set("test_support") {
   testonly = true
 
   sources = [
     "cast_metrics_test_helper.cc",
     "cast_metrics_test_helper.h",
+    "mock_cast_metrics_helper.cc",
+    "mock_cast_metrics_helper.h",
   ]
 
   public_deps = [
     ":metrics",
   ]
 
   deps = [
     "//base",
+    "//testing/gmock",
   ]
 }