Add BUILD.gn files for chromecast/media

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 = [
+    "//chromecast/base",
+  ]
+
+  configs += [ "//chromecast:config" ]
+}
+
+source_set("test_support") {
+  testonly = true
+
+  sources = [
+    "cast_metrics_test_helper.cc",
+    "cast_metrics_test_helper.h",
+  ]
+
+  deps = [
+    ":metrics",
+    "//chromecast/base",
+  ]
+
+  configs += [ "//chromecast:config" ]
+}