Use TimeTicks as much as possible in startup_metric_utils.

chrome/browser/mac/mac_startup_profiler.h

Index: chrome/browser/mac/mac_startup_profiler.h
diff --git a/chrome/browser/mac/mac_startup_profiler.h b/chrome/browser/mac/mac_startup_profiler.h
index 05dcf6cc245e77641153213318fcc7f2d39b5b5e..a91c98f0275dbc6268b318f59731f576ab74b828 100644
--- a/chrome/browser/mac/mac_startup_profiler.h
+++ b/chrome/browser/mac/mac_startup_profiler.h
@@ -50,8 +50,9 @@ class MacStartupProfiler {
   // Records UMA metrics for a specific location.
   void RecordHistogram(Location location, const base::TimeDelta& delta);
 
-  // Keeps track of the time at which each initialization phase was reached.
-  std::map<Location, base::Time> profiled_times_;
+  // Keeps track of the TimeTicks at which each initialization phase was
+  // reached.
+  std::map<Location, base::TimeTicks> profiled_ticks_;
 
   // Whether UMA metrics have been recorded. Only record UMA metrics once.
   bool recorded_metrics_;