Add UMA metric: Startup.BrowserMainToRendererMain.

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 2b2885f61a9141bbad9ecf0bacc1b6c98ee5e281..0422f4f79451bd90819f6e05b498375c5741f6d5 100644
--- a/components/startup_metric_utils/browser/startup_metric_utils.h
+++ b/components/startup_metric_utils/browser/startup_metric_utils.h
@@ -72,6 +72,12 @@ 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 a renderer main entry time. The value provided for the first
+// call to this function is used to compute
+// Startup.LoadTime.BrowserMainToRendererMain. Further calls to this
+// function are ignored.
+void RecordRendererMainEntryTime(const base::TimeTicks& ticks);
+
 // 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::TimeTicks& ticks);