Add Startup.BrowserView.FirstPaint / .CompositingEnded histograms.

chrome/browser/ui/cocoa/browser_window_cocoa.mm

Index: chrome/browser/ui/cocoa/browser_window_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_cocoa.mm b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
index 2991e80ab6535094deb95aa54984a6c72c13f3b9..45f041772b6462f24a1656fa88b6afe158906af2 100644
--- a/chrome/browser/ui/cocoa/browser_window_cocoa.mm
+++ b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
@@ -26,6 +26,7 @@
 #include "chrome/browser/ui/browser_commands.h"
 #include "chrome/browser/ui/browser_dialogs.h"
 #include "chrome/browser/ui/browser_list.h"
+#include "chrome/browser/ui/browser_view_histogram_helper.h"
 #include "chrome/browser/ui/browser_window_state.h"
 #import "chrome/browser/ui/cocoa/autofill/save_card_bubble_view_bridge.h"
 #import "chrome/browser/ui/cocoa/browser/exclusive_access_controller_views.h"
@@ -175,6 +176,10 @@ void BrowserWindowCocoa::Show() {
     // substantial part of startup time when any CALayers are part of the
     // window's NSView heirarchy.
     [window() makeKeyAndOrderFront:controller_];
+
+    // At this point all the Browser's UI is painted on the screen. There's no
+    // need to wait for the compositor, so pass the nullptr instead.
+    BrowserViewHistogramHelper().OnDidPaintChildren(nullptr);
   }
 
   // When creating windows from nibs it is necessary to |makeKeyAndOrderFront:|