Add UMA for first app list paint on Windows.

ui/app_list/views/app_list_view.cc

Index: ui/app_list/views/app_list_view.cc
diff --git a/ui/app_list/views/app_list_view.cc b/ui/app_list/views/app_list_view.cc
index 25085e968c82b1abc07d710c2d3d6bf9cbcc2a1f..29b3f7ce647523a2323819928754de1ac01c0752 100644
--- a/ui/app_list/views/app_list_view.cc
+++ b/ui/app_list/views/app_list_view.cc
@@ -4,6 +4,7 @@
 
 #include "ui/app_list/views/app_list_view.h"
 
+#include "base/callback.h"
 #include "base/command_line.h"
 #include "base/strings/string_util.h"
 #include "ui/app_list/app_list_constants.h"
@@ -37,6 +38,8 @@ namespace app_list {
 
 namespace {
 
+base::Closure g_next_paint_callback;
+
 // The distance between the arrow tip and edge of the anchor view.
 const int kArrowOffset = 10;
 
@@ -163,6 +166,14 @@ gfx::Size AppListView::GetPreferredSize() {
   return app_list_main_view_->GetPreferredSize();
 }
 
+void AppListView::Paint(gfx::Canvas* canvas) {
+  views::BubbleDelegateView::Paint(canvas);
+  if (!g_next_paint_callback.is_null()) {
+    g_next_paint_callback.Run();
+    g_next_paint_callback.Reset();
+  }
+}
+
 bool AppListView::ShouldHandleSystemCommands() const {
   return true;
 }
@@ -185,6 +196,11 @@ void AppListView::RemoveObserver(Observer* observer) {
   observers_.RemoveObserver(observer);
 }
 
+// static
+void AppListView::SetNextPaintCallback(const base::Closure& callback) {
+  g_next_paint_callback = callback;
+}
+
 #if defined(OS_WIN)
 HWND AppListView::GetHWND() const {
 #if defined(USE_AURA)