Add UMA metric of time to start the first render process

chrome/browser/metrics/startup_metrics_browsertest.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <set>
 
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/run_loop.h"
 #include "chrome/test/base/in_process_browser_test.h"
 
 using StartupMetricsTest = InProcessBrowserTest;
 
 namespace {
 
 constexpr const char* kStartupMetrics[] = {
     "Startup.BrowserMainToRendererMain",
     "Startup.BrowserOpenTabs",
     "Startup.BrowserWindow.FirstPaint",
     "Startup.BrowserWindow.FirstPaint.CompositingEnded",
     "Startup.BrowserWindowDisplay",
     "Startup.FirstWebContents.MainFrameLoad2",
     "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.
     // "Startup.BrowserMessageLoopStartHardFaultCount",
     // "Startup.BrowserMessageLoopStartTime",
     // "Startup.BrowserMessageLoopStartTimeFromMainEntry2",
     // "Startup.LoadTime.ExeMainToDllMain2",
     // "Startup.LoadTime.ProcessCreateToDllMain2",
     // "Startup.LoadTime.ProcessCreateToExeMain2",
     // "Startup.SystemUptime",
     // "Startup.Temperature",
 };
 
 }  // namespace
 
 // Verify that startup histograms are logged on browser startup.
 IN_PROC_BROWSER_TEST_F(StartupMetricsTest, ReportsValues) {
   for (auto* const histogram : kStartupMetrics) {
     while (!base::StatisticsRecorder::FindHistogram(histogram))
       base::RunLoop().RunUntilIdle();
   }
 }