Add UMA metric: Startup.BrowserMainToRendererMain.

components/startup_metric_utils/browser/startup_metric_utils.cc

Index: components/startup_metric_utils/browser/startup_metric_utils.cc
diff --git a/components/startup_metric_utils/browser/startup_metric_utils.cc b/components/startup_metric_utils/browser/startup_metric_utils.cc
index 3969ed8a94cc70ee552aa861e1943d6f5493c887..8e38d8a9308438fa5c07b5defd0e68009b3cfe8c 100644
--- a/components/startup_metric_utils/browser/startup_metric_utils.cc
+++ b/components/startup_metric_utils/browser/startup_metric_utils.cc
@@ -29,7 +29,10 @@ volatile bool g_non_browser_ui_displayed = false;
 base::LazyInstance<base::Time>::Leaky g_process_creation_time =
     LAZY_INSTANCE_INITIALIZER;
 
-base::LazyInstance<base::Time>::Leaky g_main_entry_point_time =
+base::LazyInstance<base::Time>::Leaky g_browser_main_entry_point_time =
+    LAZY_INSTANCE_INITIALIZER;
+
+base::LazyInstance<base::Time>::Leaky g_renderer_main_entry_point_time =
     LAZY_INSTANCE_INITIALIZER;
 
 StartupTemperature g_startup_temperature = UNCERTAIN_STARTUP_TEMPERATURE;
@@ -163,6 +166,8 @@ bool GetHardFaultCountForCurrentProcess(uint32_t* hard_fault_count,
 // ".ColdStart" or ".WarmStart", as appropriate.
 // |value_expr| is an expression evaluating to the value to be recorded. This
 // will be evaluated exactly once and cached, so side effects are not an issue.
+// A metric logged using this macro must have an affected-histogram entry in the
+// definition of the StartupTemperature suffix in histograms.xml.
 #define UMA_HISTOGRAM_WITH_STARTUP_TEMPERATURE(type, basename, value_expr) \
   {                                                                        \
     const auto kValue = value_expr;                                        \
@@ -290,6 +295,21 @@ void RecordMainEntryTimeHistogram() {
       browser_main_entry_time_raw_ms_low_word);
 }
 
+// Record renderer main entry time histogram.
+void RecordRendererMainEntryHistogram() {
+  const base::Time& browser_main_entry_point_time =
+      g_browser_main_entry_point_time.Get();
+  const base::Time& renderer_main_entry_point_time =
+      g_renderer_main_entry_point_time.Get();
+
+  if (!browser_main_entry_point_time.is_null() &&
+      !renderer_main_entry_point_time.is_null()) {
+    UMA_HISTOGRAM_AND_TRACE_WITH_STARTUP_TEMPERATURE(
+        UMA_HISTOGRAM_LONG_TIMES_100, "Startup.BrowserMainToRendererMain",
+        browser_main_entry_point_time, renderer_main_entry_point_time);
+  }
+}
+
 // Environment variable that stores the timestamp when the executable's main()
 // function was entered.
 const char kChromeMainTimeEnvVar[] = "CHROME_MAIN_TIME";
@@ -324,7 +344,7 @@ void RecordStartupProcessCreationTime(const base::Time& time) {
 
 void RecordMainEntryPointTime(const base::Time& time) {
   DCHECK(MainEntryPointTime().is_null());
-  g_main_entry_point_time.Get() = time;
+  g_browser_main_entry_point_time.Get() = time;
   DCHECK(!MainEntryPointTime().is_null());
 }
 
@@ -417,6 +437,13 @@ void RecordBrowserOpenTabsDelta(const base::TimeDelta& delta) {
                                          "Startup.BrowserOpenTabs", delta);
 }
 
+void RecordRendererMainEntryTime(const base::Time& time) {
+  // Record the renderer main entry time, but don't log the UMA metric
+  // immediately because the startup temperature is not known yet.
+  if (g_renderer_main_entry_point_time.Get().is_null())
+    g_renderer_main_entry_point_time.Get() = time;
+}
+
 void RecordFirstWebContentsMainFrameLoad(const base::Time& time) {
   static bool is_first_call = true;
   if (!is_first_call || time.is_null())
@@ -448,6 +475,11 @@ void RecordFirstWebContentsNonEmptyPaint(const base::Time& time) {
   if (!is_first_call || time.is_null())
     return;
   is_first_call = false;
+
+  // Log Startup.BrowserMainToRendererMain now that the first renderer main
+  // entry time and the startup temperature are known.
+  RecordRendererMainEntryHistogram();
+
   if (WasNonBrowserUIDisplayed() || g_process_creation_time.Get().is_null())
     return;
 
@@ -498,7 +530,7 @@ void RecordFirstWebContentsMainNavigationFinished(const base::Time& time) {
 }
 
 base::Time MainEntryPointTime() {
-  return g_main_entry_point_time.Get();
+  return g_browser_main_entry_point_time.Get();
 }
 
 StartupTemperature GetStartupTemperature() {