Added UMA metrics for screen capture UI in the status area.

ash/common/system/chromeos/screen_security/screen_tray_item.cc

Index: ash/common/system/chromeos/screen_security/screen_tray_item.cc
diff --git a/ash/common/system/chromeos/screen_security/screen_tray_item.cc b/ash/common/system/chromeos/screen_security/screen_tray_item.cc
index d04f10ac30cddb461c54e22d21fa515156699708..0940bcfc0fa657e91131f08db8dca5a87ba9c7cc 100644
--- a/ash/common/system/chromeos/screen_security/screen_tray_item.cc
+++ b/ash/common/system/chromeos/screen_security/screen_tray_item.cc
@@ -77,6 +77,7 @@ void ScreenStatusView::ButtonPressed(views::Button* sender,
                                      const ui::Event& event) {
   DCHECK(sender == stop_button_);
   screen_tray_item_->Stop();
+  screen_tray_item_->RecordStoppedFromDefaultViewMetric();
 }
 
 void ScreenStatusView::CreateItems() {
@@ -113,6 +114,7 @@ ScreenNotificationDelegate::~ScreenNotificationDelegate() {}
 void ScreenNotificationDelegate::ButtonClick(int button_index) {
   DCHECK_EQ(0, button_index);
   screen_tray_->Stop();
+  screen_tray_->RecordStoppedFromNotificationViewMetric();
 }
 
 }  // namespace tray
@@ -167,6 +169,10 @@ void ScreenTrayItem::Stop() {
   callback.Run();
 }
 
+void ScreenTrayItem::RecordStoppedFromDefaultViewMetric() {}
+
+void ScreenTrayItem::RecordStoppedFromNotificationViewMetric() {}
+
 void ScreenTrayItem::DestroyTrayView() {
   tray_view_ = NULL;
 }