implement SetWatchEvent and WaitForEvent for trace-based-tests

content/public/browser/trace_subscriber.h

Index: content/public/browser/trace_subscriber.h
diff --git a/content/public/browser/trace_subscriber.h b/content/public/browser/trace_subscriber.h
index 08e408c1b80b83b218071fc1cbb694d7200bfa15..ec8d070ba4b8fc00e526aa0f5935972d0f05522d 100644
--- a/content/public/browser/trace_subscriber.h
+++ b/content/public/browser/trace_subscriber.h
@@ -11,7 +11,8 @@
 
 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 {
@@ -31,6 +32,8 @@ class TraceSubscriber {
 
   virtual void OnTraceBufferPercentFullReply(float percent_full) {}
 
+  virtual void OnEventWatchNotification() {}
+
  protected:
   virtual ~TraceSubscriber() {}
 };