Add UMA for startup load time of component extensions. At the moment there is

chrome/browser/extensions/extension_service.cc

Index: chrome/browser/extensions/extension_service.cc
diff --git a/chrome/browser/extensions/extension_service.cc b/chrome/browser/extensions/extension_service.cc
index a93594708fc816d00a30ee1df19b28ffd2125a3f..92cf251f424b90b1cf1f364be6f4557a99d29c7b 100644
--- a/chrome/browser/extensions/extension_service.cc
+++ b/chrome/browser/extensions/extension_service.cc
@@ -1408,11 +1408,14 @@ void ExtensionService::ReloadExtensionsForTest() {
 }
 
 void ExtensionService::SetReadyAndNotifyListeners() {
+  base::TimeTicks start_time = base::TimeTicks::Now();

[Mark P, 2015/01/28 21:30:12]

nit: might as well be const

   ready_->Signal();
   content::NotificationService::current()->Notify(
       extensions::NOTIFICATION_EXTENSIONS_READY_DEPRECATED,
       content::Source<Profile>(profile_),
       content::NotificationService::NoDetails());
+  UMA_HISTOGRAM_TIMES("Extensions.ExtensionServiceNotifyReadyListenersTime",
+                      base::TimeTicks::Now() - start_time);
 }
 
 void ExtensionService::OnLoadedInstalledExtensions() {