Added UMA stats to overview mode

ash/wm/overview/window_grid.cc

Index: ash/wm/overview/window_grid.cc
diff --git a/ash/wm/overview/window_grid.cc b/ash/wm/overview/window_grid.cc
index e59d078bf3accc0c87fd28249a6d8fe57e7d872c..ced57d57e31c7b07874620e2352b414f3e06f702 100644
--- a/ash/wm/overview/window_grid.cc
+++ b/ash/wm/overview/window_grid.cc
@@ -308,6 +308,9 @@ void WindowGrid::OnWindowDestroying(aura::Window* window) {
   size_t removed_index = iter - window_list_.begin();
   window_list_.erase(iter);
 
+  // Record that an item has been removed for statistical purposes.
+  window_selector_->on_window_removed();
+
   if (empty()) {
     // If the grid is now empty, notify the window selector so that it erases us
     // from its grid list.