Use TimeTicks as much as possible in startup_metric_utils.

mojo/services/tracing/public/interfaces/tracing.mojom

Index: mojo/services/tracing/public/interfaces/tracing.mojom
diff --git a/mojo/services/tracing/public/interfaces/tracing.mojom b/mojo/services/tracing/public/interfaces/tracing.mojom
index b702163ecf26cb6db13d7c2e8c6c2fbe9926b509..b624f81e24f9da19b3c352d8b9c21b2e80bb8332 100644
--- a/mojo/services/tracing/public/interfaces/tracing.mojom
+++ b/mojo/services/tracing/public/interfaces/tracing.mojom
@@ -31,18 +31,19 @@ interface TraceCollector {
 };
 
 // These times are used to determine startup performance metrics.
-// TODO(msw): Use TimeTicks to avoid system clock changes: crbug.com/521164
+// TODO(msw): Find a way to convert *_time metrics into TimeTicks earlier (ref:
+// https://goo.gl/vZ8dZW).
 struct StartupPerformanceTimes {
   // TODO(msw): Rename to match "BrowserMainEntryTimeAbsolute" metric?
   int64 shell_process_creation_time;
   int64 shell_main_entry_point_time;
-  int64 browser_message_loop_start_time;
-  int64 browser_window_display_time;
+  int64 browser_message_loop_start_ticks;
+  int64 browser_window_display_ticks;
   int64 browser_open_tabs_time_delta;
   // TODO(msw): Rename to avoid "web contents"?
-  int64 first_web_contents_main_frame_load_time;
+  int64 first_web_contents_main_frame_load_ticks;
   // TODO(msw): Rename to match "FirstWebContents.NonEmptyPaint" metric?
-  int64 first_visually_non_empty_layout_time;
+  int64 first_visually_non_empty_layout_ticks;
 };
 
 // This interface accepts startup performance timing from a variety of sources.
@@ -50,11 +51,11 @@ interface StartupPerformanceDataCollector {
   // These setters may be called many times, only the first time is recorded.
   SetShellProcessCreationTime(int64 time);
   SetShellMainEntryPointTime(int64 time);
-  SetBrowserMessageLoopStartTime(int64 time);
-  SetBrowserWindowDisplayTime(int64 time);
+  SetBrowserMessageLoopStartTicks(int64 ticks);
+  SetBrowserWindowDisplayTicks(int64 ticks);
   SetBrowserOpenTabsTimeDelta(int64 delta);
-  SetFirstWebContentsMainFrameLoadTime(int64 time);
-  SetFirstVisuallyNonEmptyLayoutTime(int64 time);
+  SetFirstWebContentsMainFrameLoadTicks(int64 ticks);
+  SetFirstVisuallyNonEmptyLayoutTicks(int64 ticks);
 
   // Get the currently available startup performance times.
   GetStartupPerformanceTimes() => (StartupPerformanceTimes times);