[Media Router] Reland: MediaRouter interfaces with stub implementations.

chrome/browser/media/router/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.
 #
 import("//testing/test.gni")
 
 source_set("router") {
   deps = [
+    ":router_core",
+    ":keyed_service_factories",
+  ]
+}
+
+source_set("router_core") {
+  deps = [
     "//base",
+    "//components/keyed_service/core",
     "//url",
   ]
   sources = [
     "media_route.cc",
     "media_route.h",
     "media_route_id.h",
+    "media_router.h",
+    "media_router_impl.cc",
+    "media_router_impl.h",
+    "media_routes_observer.cc",
+    "media_routes_observer.h",
     "media_sink.cc",
     "media_sink.h",
+    "media_sinks_observer.cc",
+    "media_sinks_observer.h",
     "media_source.cc",
     "media_source.h",
     "media_source_helper.cc",
     "media_source_helper.h",
     "route_id_manager.cc",
     "route_id_manager.h",
   ]
 }
 
+source_set("keyed_service_factories") {
+  deps = [
+    ":router_core",
+    "//components/keyed_service/content",
+  ]
+  sources = [
+    "media_router_impl_factory.cc",
+    "media_router_impl_factory.h",
+  ]
+}
+
 source_set("unit_tests") {
   testonly = true
   deps = [
-    ":router",
+    ":router_core",
     "//base/test:test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
   sources = [
     "media_route_unittest.cc",
     "media_sink_unittest.cc",
     "media_source_unittest.cc",
     "route_id_manager_unittest.cc",
   ]
 }
 
 # Optional standalone test binary, for faster isolated builds.
 test("unit_tests_main") {
   deps = [
     ":unit_tests",
     "//testing/gmock:gmock_main",
   ]
   sources = [
     ":unittest_files",
   ]
 }