Add media router common classes and unittests.

chrome/browser/media/router/BUILD.gn

Index: chrome/browser/media/router/BUILD.gn
diff --git a/chrome/browser/media/router/BUILD.gn b/chrome/browser/media/router/BUILD.gn
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+# 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.
+
+# Combined build target for all non-UI media router code.
+source_set("media_router") {
+  deps = [
+    ":defines",
+  ]
+  # No "sources" for this target - add submodules to "deps" instead.
+}
+
+source_set("defines") {
+  deps = [
+    "//base",
+  ]
+  sources = [
+    "media_router_defines.h",
+  ]
+}