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

mojo/services/tracing/tracing.mojom

Index: mojo/services/tracing/tracing.mojom
diff --git a/mojo/services/tracing/tracing.mojom b/mojo/services/tracing/tracing.mojom
index fa8be1f655df695b43a8a17d294dd40f74b30a54..50dc5e065cdbf9619f8ea676827ce614e30a6e0e 100644
--- a/mojo/services/tracing/tracing.mojom
+++ b/mojo/services/tracing/tracing.mojom
@@ -26,3 +26,28 @@ interface TraceCoordinator {
   // Stop tracing and flush results to file.
   StopAndFlush();
 };
+
+// These times are used to determine startup performance metrics.
+struct StartupPerformanceTimes {
+  // TODO(msw): Rename to match "BrowserMainEntryTimeAbsolute" metric?
+  int64 shell_process_creation_time;

[sky, 2015/08/12 23:54:36]

Again, I think all this should be timeticks, not times.

[msw, 2015/08/14 23:20:35]

I filed http://crbug.com/521164 for this and added TODOs.

+  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 StartupPerformanceController {

[yzshen1, 2015/08/13 15:59:12]

'Controller' seems a little confusing. Because Trace'Controller' above is an
interface implemented by clients of the tracing app.
StartupPerformance'Controller', on the other hand, is implemented by the tracing
app.

This interface is more like the TraceDataCollector above. Maybe it should be
called StartupPerformanceDataCollector?

[msw, 2015/08/14 23:20:35]

I went with StartupPerformanceDataCollector, but it's a long name; other
suggestions are welcome.

[yzshen1, 2015/08/17 05:46:38]

(Optional) One idea is "Performance" -> "Perf". So it is
StartupPerfDataCollector.

+  // 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);
+};