Add Startup.BrowserView.FirstPaint / .CompositingEnded histograms.

chrome/browser/background/background_mode_manager.cc

Index: chrome/browser/background/background_mode_manager.cc
diff --git a/chrome/browser/background/background_mode_manager.cc b/chrome/browser/background/background_mode_manager.cc
index 56524fb60a19144150520ce95f584c55c3c07e20..096064f033b1e7905c595ed9e4cfb814a4b1e538 100644
--- a/chrome/browser/background/background_mode_manager.cc
+++ b/chrome/browser/background/background_mode_manager.cc
@@ -57,6 +57,7 @@
 #include "chrome/grit/generated_resources.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/pref_service.h"
+#include "components/startup_metric_utils/browser/startup_metric_utils.h"
 #include "content/public/browser/notification_service.h"
 #include "extensions/browser/extension_system.h"
 #include "extensions/common/constants.h"
@@ -717,6 +718,8 @@ void BackgroundModeManager::StartBackgroundMode() {
   if (in_background_mode_)
     return;
 
+  startup_metric_utils::SetBackgroundModeEnabled();

[sky, 2017/04/11 17:13:36]

How come you don't need a matching End?

[themblsha, 2017/04/12 17:18:28]

The intent is to set this once per startup, as it could delay the normal
appearance of the first Browser window. The only way to 'end' this is to restart
the browser.

It would affect the unit_tests but they're out of scope of this problem anyway
since startup_metric_utils.cc already rely on global singletons and require the
tests to be inside browser_tests / interactive_ui_tests anyway.

+
   // Mark ourselves as running in background mode.
   in_background_mode_ = true;