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

mojo/services/tracing/public/interfaces/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();

[sky, 2015/08/17 20:09:30]

How do you know when the final DataCollected() is called?

[msw, 2015/08/17 21:25:51]

I just added the struct and interface below and moved this file from
mojo/services/tracing/. I don't know much about the existing (unchanged)
interfaces.

+};
+
+// These times are used to determine startup performance metrics.
+// TODO(msw): Use TimeTicks to avoid system clock changes: crbug.com/521164
+struct StartupPerformanceTimes {
+  // TODO(msw): Rename to match "BrowserMainEntryTimeAbsolute" metric?
+  int64 shell_process_creation_time;
+  int64 browser_message_loop_start_time;
+  int64 browser_window_display_time;
+  int64 browser_open_tabs_time;
+  // TODO(msw): Rename to avoid "web contents"?
+  int64 first_web_contents_main_frame_load_time;
+  // TODO(msw): Rename to match "FirstWebContents.NonEmptyPaint" metric?
+  int64 first_visually_non_empty_layout_time;
+};
+
+interface StartupPerformanceDataCollector {

[sky, 2015/08/17 20:09:30]

Expand your documentation above to cover this.
It's also not clear why there are a bunch of setters. Could a single Done() with
a struct be used?

[msw, 2015/08/17 21:25:51]

I added minimal documentation, let me know if it's okay. A single setter
wouldn't work, as we need data from the shell (via context.cc), mojo:browser,
and mojo:html_viewer. I considered using a single setter function that took a
string and a value, eg. SetTracingTime("ShellProcessCreation", time), and then
the collector could just return a map of string->time... would you prefer that?

[sky, 2015/08/17 21:41:57]

No, that's too obscure. If you can't do the single setter than I prefer what you
have here.

+  // These setters may be called many times, only the first call is recorded.
+  SetShellProcessCreationTime(int64 time);
+  SetBrowserMessageLoopStartTime(int64 time);
+  SetBrowserWindowDisplayTime(int64 time);
+  SetBrowserOpenTabsTime(int64 time);
+  SetFirstWebContentsMainFrameLoadTime(int64 time);
+  SetFirstVisuallyNonEmptyLayoutTime(int64 time);
+
+  GetStartupPerformanceTimes() => (StartupPerformanceTimes times);
+};