Use TimeTicks as much as possible in startup_metric_utils.

components/startup_metric_utils/browser/startup_metric_utils.h

Index: components/startup_metric_utils/browser/startup_metric_utils.h
diff --git a/components/startup_metric_utils/browser/startup_metric_utils.h b/components/startup_metric_utils/browser/startup_metric_utils.h
index b071321b73a1b80ddf1f8050ed3ec36dced300b1..2b2885f61a9141bbad9ecf0bacc1b6c98ee5e281 100644
--- a/components/startup_metric_utils/browser/startup_metric_utils.h
+++ b/components/startup_metric_utils/browser/startup_metric_utils.h
@@ -9,7 +9,11 @@
 
 #include "base/time/time.h"
 
-// Utility functions to support metric collection for browser startup.
+// Utility functions to support metric collection for browser startup. Timings
+// should use TimeTicks whenever possible. OS-provided timings are still
+// received as Time out of cross-platform support necessity but are converted to
+// TimeTicks as soon as possible in an attempt to reduce the potential skew
+// between the two basis. See crbug.com/544131 for reasoning.
 
 namespace startup_metric_utils {
 
@@ -59,43 +63,45 @@ void RecordExeMainEntryPointTime(const base::Time& time);
 
 // Call this with the time recorded just before the message loop is started.
 // |is_first_run| - is the current launch part of a first run.
-void RecordBrowserMainMessageLoopStart(const base::Time& time,
+void RecordBrowserMainMessageLoopStart(const base::TimeTicks& ticks,
                                        bool is_first_run);
 
 // Call this with the time when the first browser window became visible.
-void RecordBrowserWindowDisplay(const base::Time& time);
+void RecordBrowserWindowDisplay(const base::TimeTicks& ticks);
 
 // Call this with the time delta that the browser spent opening its tabs.
 void RecordBrowserOpenTabsDelta(const base::TimeDelta& delta);
 
 // Call this with the time when the first web contents loaded its main frame,
 // only if the first web contents was unimpended in its attempt to do so.
-void RecordFirstWebContentsMainFrameLoad(const base::Time& time);
+void RecordFirstWebContentsMainFrameLoad(const base::TimeTicks& ticks);
 
 // Call this with the time when the first web contents loaded its main frame.
 // This records an old stat kept for comparison purposes until M49.
-void RecordDeprecatedFirstWebContentsMainFrameLoad(const base::Time& time);
+void RecordDeprecatedFirstWebContentsMainFrameLoad(
+    const base::TimeTicks& ticks);
 
 // Call this with the time when the first web contents had a non-empty paint,
 // only if the first web contents was unimpended in its attempt to do so.
-void RecordFirstWebContentsNonEmptyPaint(const base::Time& time);
+void RecordFirstWebContentsNonEmptyPaint(const base::TimeTicks& ticks);
 
 // Call this with the time when the first web contents had a non-empty paint.
 // This records an old stat kept for comparison purposes until M49.
-void RecordDeprecatedFirstWebContentsNonEmptyPaint(const base::Time& time);
+void RecordDeprecatedFirstWebContentsNonEmptyPaint(
+    const base::TimeTicks& ticks);
 
 // Call this with the time when the first web contents began navigating its main
 // frame.
-void RecordFirstWebContentsMainNavigationStart(const base::Time& time);
+void RecordFirstWebContentsMainNavigationStart(const base::TimeTicks& ticks);
 
 // Call this with the time when the first web contents successfully committed
 // its navigation for the main frame.
-void RecordFirstWebContentsMainNavigationFinished(const base::Time& time);
+void RecordFirstWebContentsMainNavigationFinished(const base::TimeTicks& ticks);
 
-// Returns the time of main entry recorded from RecordMainEntryPointTime.
-// Returns a null Time if a value has not been recorded yet.
-// This method is expected to be called from the UI thread.
-base::Time MainEntryPointTime();
+// Returns the TimeTicks corresponding to main entry as recorded by
+// RecordMainEntryPointTime. Returns a null TimeTicks if a value has not been
+// recorded yet. This method is expected to be called from the UI thread.
+base::TimeTicks MainEntryPointTicks();
 
 // Returns the startup type. This is only currently supported on the Windows
 // platform and will simply return UNCERTAIN_STARTUP_TYPE on other platforms.