Fix a tyop: "particate" -> "participate".

services/tracing/tracing.mojom

 // Copyright 2014 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.
 
 module tracing;
 
-// To particate in the tracing ecosystem, implement the TraceController
+// To participate in the tracing ecosystem, implement the TraceController
 // interface and connect to the tracing app. Then, when the controller's Start()
 // function is called collect tracing data and pass it back via the provided
 // TraceDataCollector interface up until Stop() is called.
 
 interface TraceController {
   StartTracing(string categories, TraceDataCollector collector);
   StopTracing();
 };
 
 interface TraceDataCollector {
   DataCollected(string json);
 };
 
 interface TraceCoordinator {
   // Request tracing data from all connected TraceControllers to stream to
   // |stream|.
   Start(handle<data_pipe_producer> stream, string categories);
 
   // Stop tracing and flush results to file.
   StopAndFlush();
 };