Add UMA metric of time to start the first render process

chrome/browser/metrics/startup_metrics_browsertest.cc

Index: chrome/browser/metrics/startup_metrics_browsertest.cc
diff --git a/chrome/browser/metrics/startup_metrics_browsertest.cc b/chrome/browser/metrics/startup_metrics_browsertest.cc
index 71932aff5228498d725ce49fc46736a3fe1b4d88..ab9258f07b2ee6c6b09965708dc0714457ded38c 100644
--- a/chrome/browser/metrics/startup_metrics_browsertest.cc
+++ b/chrome/browser/metrics/startup_metrics_browsertest.cc
@@ -23,6 +23,7 @@ constexpr const char* kStartupMetrics[] = {
     "Startup.FirstWebContents.MainNavigationFinished",
     "Startup.FirstWebContents.MainNavigationStart",
     "Startup.FirstWebContents.NonEmptyPaint2",
+    "Startup.FirstWebContents.RenderProcessHostInit.ToNonEmptyPaint",
 
     // The following histograms depend on normal browser startup through
     // BrowserMain and are as such not caught by this browser test.