Move pc/media_stream_track.h to the api/ directory

This file is being accessed from Chrome. Moving it lessens the
dependency of Chrome on files in the pc/ directory, and allows
easier refactoring of pc/.

Bug: webrtc:11967
Change-Id: Iccd568f84e9cf4086e37c58db1b4cba6c376f413
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187489
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32378}
diff --git a/pc/audio_track.h b/pc/audio_track.h
index a0388e8..8cff79e 100644
--- a/pc/audio_track.h
+++ b/pc/audio_track.h
@@ -14,8 +14,8 @@
 #include <string>
 
 #include "api/media_stream_interface.h"
+#include "api/media_stream_track.h"
 #include "api/scoped_refptr.h"
-#include "pc/media_stream_track.h"
 #include "rtc_base/thread_checker.h"
 
 namespace webrtc {