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

services/video_capture/public/cpp/constants.h

Index: services/video_capture/public/cpp/constants.h
diff --git a/services/video_capture/public/cpp/constants.h b/services/video_capture/public/cpp/constants.h
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+++ b/services/video_capture/public/cpp/constants.h
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+// 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.
+
+#ifndef SERVICES_VIDEO_CAPTURE_PUBLIC_CPP_CONSTANTS_H_
+#define SERVICES_VIDEO_CAPTURE_PUBLIC_CPP_CONSTANTS_H_
+
+#include "base/feature_list.h"
+
+namespace video_capture {
+
+extern const base::Feature kMojoVideoCapture;
+
+}  // namespace video_capture
+
+#endif  // SERVICES_VIDEO_CAPTURE_PUBLIC_CPP_CONSTANTS_H_