(5) Rename files to snake_case: install forwarding headers

Mechanically generated with this command:

tools_webrtc/do-renames.sh install all-renames.txt && git cl format

Bug: webrtc:10159
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ic8e99f71f2da62e5c99863c6d24a8cfe311466cd
Reviewed-on: https://webrtc-review.googlesource.com/c/115682
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26227}

api/mediastreaminterface.h

@@ -0,0 +1,19 @@
+/*
+ *  Copyright 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef API_MEDIASTREAMINTERFACE_H_
+#define API_MEDIASTREAMINTERFACE_H_
+
+// TODO(bugs.webrtc.org/10159): Remove this files once downstream projects have
+// been updated to include the new path.
+
+#include "api/media_stream_interface.h"
+
+#endif  // API_MEDIASTREAMINTERFACE_H_