Move RtcEventLogOutput to api/

Move RtcEventLogOutput into the API, so that we would be able to change StartRtcEventLog (in PeerConnectionInterface) to use it.

Bug: webrtc:8111
Change-Id: I1d70af792ec584d3f1a8eced1b66c38e4a360642
Reviewed-on: https://webrtc-review.googlesource.com/7220
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20189}
diff --git a/api/rtceventlogoutput.h b/api/rtceventlogoutput.h
new file mode 100644
index 0000000..67e408d
--- /dev/null
+++ b/api/rtceventlogoutput.h
@@ -0,0 +1,39 @@
+/*
+ *  Copyright (c) 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.
+ */
+
+#ifndef API_RTCEVENTLOGOUTPUT_H_
+#define API_RTCEVENTLOGOUTPUT_H_
+
+#include <string>
+
+namespace webrtc {
+
+// NOTE: This class is still under development and may change without notice.
+class RtcEventLogOutput {
+ public:
+  virtual ~RtcEventLogOutput() = default;
+
+  // An output normally starts out active, though that might not always be
+  // the case (e.g. failed to open a file for writing).
+  // Once an output has become inactive (e.g. maximum file size reached), it can
+  // never become active again.
+  virtual bool IsActive() const = 0;
+
+  // Write encoded events to an output. Returns true if the output was
+  // successfully written in its entirety. Otherwise, no guarantee is given
+  // about how much data was written, if any. The output sink becomes inactive
+  // after the first time |false| is returned. Write() may not be called on
+  // an inactive output sink.
+  virtual bool Write(const std::string& output) = 0;
+};
+
+}  // namespace webrtc
+
+#endif  // API_RTCEVENTLOGOUTPUT_H_