[Mojo Video Capture] Hook up video capture service behind a feature flag

services/video_capture/public/cpp/BUILD.gn

 # Copyright 2017 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("cpp") {
   sources = [
+    "constants.cc",
+    "constants.h",
     "device_to_feedback_observer_adapter.cc",
     "device_to_feedback_observer_adapter.h",
     "receiver_media_to_mojo_adapter.cc",
     "receiver_media_to_mojo_adapter.h",
   ]
 
   public_deps = [
     "//base",
     "//media",
     "//media/capture:capture",
     "//services/service_manager/public/cpp",
     "//services/video_capture/public/interfaces",
   ]
 
   deps = [
     "//mojo/public/cpp/bindings:bindings",
   ]
 }