Add stats collection for telemetry startup.warm.blank_page test.

mojo/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 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();
-};