Revert of Remove remains of webrtc/base (patchset #7 id:120001 of https://codereview.webrtc.org/297…

webrtc/base/event_tracer.h

+/*
+ *  Copyright (c) 2012 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.
+ */
+
+// This file defines the interface for event tracing in WebRTC.
+//
+// Event log handlers are set through SetupEventTracer(). User of this API will
+// provide two function pointers to handle event tracing calls.
+//
+// * GetCategoryEnabledPtr
+//   Event tracing system calls this function to determine if a particular
+//   event category is enabled.
+//
+// * AddTraceEventPtr
+//   Adds a tracing event. It is the user's responsibility to log the data
+//   provided.
+//
+// Parameters for the above two functions are described in trace_event.h.
+
+#ifndef WEBRTC_BASE_EVENT_TRACER_H_
+#define WEBRTC_BASE_EVENT_TRACER_H_
+
+
+// This header is deprecated and is just left here temporarily during
+// refactoring. See https://bugs.webrtc.org/7634 for more details.
+#include "webrtc/rtc_base/event_tracer.h"
+
+#endif  // WEBRTC_BASE_EVENT_TRACER_H_