Add Startup.TimeSinceLastStartup histogram.

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 51bae624a755f0a96dae3e0bea7c61ec11fea5fb..1027792ce66e567a30d97c3f143027c2c302f976 100644
--- a/components/startup_metric_utils/browser/startup_metric_utils.h
+++ b/components/startup_metric_utils/browser/startup_metric_utils.h
@@ -11,6 +11,9 @@
 #include "base/time/time.h"
 #include "build/build_config.h"
 
+class PrefRegistrySimple;
+class PrefService;
+
 // Utility functions to support metric collection for browser startup. Timings
 // should use TimeTicks whenever possible. OS-provided timings are still
 // received as Time out of cross-platform support necessity but are converted to
@@ -42,6 +45,9 @@ enum StartupTemperature {
 bool GetHardFaultCountForCurrentProcess(uint32_t* hard_fault_count);
 #endif  // defined(OS_WIN)
 
+// Registers startup related prefs in |registry|.
+void RegisterPrefs(PrefRegistrySimple* registry);
+
 // Returns true if any UI other than the browser window has been displayed
 // so far.  Useful to test if UI has been displayed before the first browser
 // window was shown, which would invalidate any surrounding timing metrics.
@@ -74,6 +80,13 @@ void RecordExeMainEntryPointTime(const base::Time& time);
 void RecordBrowserMainMessageLoopStart(const base::TimeTicks& ticks,
                                        bool is_first_run);
 
+// Logs the Startup.TimeSinceLastStartup histogram. Obtains the timestamp of the
+// last startup from |pref_service| and overwrites it with the timestamp of the
+// current startup. If the startup temperature has been set by
+// RecordBrowserMainMessageLoopStart, the time since last startup is also logged
+// to an histogram suffixed with the startup temperature.
+void RecordTimeSinceLastStartup(PrefService* pref_service);
+
 // Call this with the time when the first browser window became visible.
 void RecordBrowserWindowDisplay(const base::TimeTicks& ticks);