Create //webrtc/api:libjingle_peerconnection_api + refactorings.

Create a new target //webrtc/api:libjingle_peerconnection_api and start moving
things into it. Move remaining parts of //webrtc/api:libjingle_peerconnection
to //webrtc/pc:libjingle_peerconnection.

Moved the RTCStatsCollectorCallback into its own header file, so that
PeerConnectionInterface can include that instead of pulling in
RTCStatsCollector and PeerConnection and everything.

Separated cricket::MediaType into its own header/source set, so that it
can be used in the api.

BUG=webrtc:5883

Review-Url: https://codereview.webrtc.org/2514883002
Cr-Commit-Position: refs/heads/master@{#16210}

webrtc/api/mediastreaminterface.cc

diff --git a/webrtc/api/mediastreaminterface.cc b/webrtc/api/mediastreaminterface.cc
new file mode 100644
index 0000000..d4fe5cb
--- /dev/null
+++ b/webrtc/api/mediastreaminterface.cc
@@ -0,0 +1,18 @@
+/*
+ *  Copyright 2017 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.
+ */
+
+#include "webrtc/api/mediastreaminterface.h"
+
+namespace webrtc {
+
+const char MediaStreamTrackInterface::kVideoKind[] = "video";
+const char MediaStreamTrackInterface::kAudioKind[] = "audio";
+
+}  // namespace webrtc