Revert of Delete unused file mediacommon.h. (patchset #1 id:1 of https://codereview.webrtc.org/2437703002/ )

Reason for revert:
Turned out this broke a downstream app. Reverting while investigating.

Original issue's description:
> Delete unused file mediacommon.h.
>
> BUG=None
>
> Committed: https://crrev.com/a34e796a1b59a1068c77a3e36d2a24fd37c08afc
> Cr-Commit-Position: refs/heads/master@{#14689}

TBR=magjed@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=None

Review-Url: https://codereview.webrtc.org/2441493003
Cr-Commit-Position: refs/heads/master@{#14697}

webrtc/media/BUILD.gn

@@ -56,6 +56,7 @@ rtc_static_library("rtc_media") {
     "base/device.h",
     "base/hybriddataengine.h",
     "base/mediachannel.h",
+    "base/mediacommon.h",
     "base/mediaconstants.cc",
     "base/mediaconstants.h",
     "base/mediaengine.cc",

webrtc/media/base/mediacommon.h

@@ -0,0 +1,27 @@
+/*
+ *  Copyright (c) 2004 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 WEBRTC_MEDIA_BASE_MEDIACOMMON_H_
+#define WEBRTC_MEDIA_BASE_MEDIACOMMON_H_
+
+#include "webrtc/base/stringencode.h"
+
+namespace cricket {
+
+enum MediaCapabilities {
+  AUDIO_RECV = 1 << 0,
+  AUDIO_SEND = 1 << 1,
+  VIDEO_RECV = 1 << 2,
+  VIDEO_SEND = 1 << 3,
+};
+
+}  // namespace cricket
+
+#endif  // WEBRTC_MEDIA_BASE_MEDIACOMMON_H_

webrtc/media/base/mediaengine.h

@@ -24,6 +24,7 @@
 #include "webrtc/base/sigslotrepeater.h"
 #include "webrtc/media/base/codec.h"
 #include "webrtc/media/base/mediachannel.h"
+#include "webrtc/media/base/mediacommon.h"
 #include "webrtc/media/base/videocommon.h"
 #include "webrtc/modules/audio_coding/codecs/audio_decoder_factory.h"
 

webrtc/media/media.gyp

@@ -35,6 +35,7 @@
         'base/device.h',
         'base/hybriddataengine.h',
         'base/mediachannel.h',
+        'base/mediacommon.h',
         'base/mediaconstants.cc',
         'base/mediaconstants.h',
         'base/mediaengine.cc',