implement SetWatchEvent and WaitForEvent for trace-based-tests

content/public/browser/trace_subscriber.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_TRACE_SUBSCRIBER_H_
 #define CONTENT_PUBLIC_BROWSER_TRACE_SUBSCRIBER_H_
 
 #include <set>
 
 #include "base/memory/ref_counted_memory.h"
 
 namespace content {
 
-// Objects interested in receiving trace data derive from TraceSubscriber.
+// Objects interested in receiving trace data derive from TraceSubscriber. All
+// callbacks occur on the UI thread.
 // See also: trace_message_filter.h
 // See also: child_trace_message_filter.h
 class TraceSubscriber {
  public:
   // Called once after TraceController::EndTracingAsync.
   virtual void OnEndTracingComplete() = 0;
 
   // Called 0 or more times between TraceController::BeginTracing and
   // OnEndTracingComplete. Use base::debug::TraceResultBuffer to convert one or
   // more trace fragments to JSON.
   virtual void OnTraceDataCollected(
       const scoped_refptr<base::RefCountedString>& trace_fragment) = 0;
 
   // Called once after TraceController::GetKnownCategoriesAsync.
   virtual void OnKnownCategoriesCollected(
       const std::set<std::string>& known_categories) {}
 
   virtual void OnTraceBufferPercentFullReply(float percent_full) {}
 
+  virtual void OnEventWatchNotification() {}
+
  protected:
   virtual ~TraceSubscriber() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_TRACE_SUBSCRIBER_H_