ash: Initialize some pointers to null in SystemTray constructor

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index a3130a965c8d5c1437f575a0bfd8ad06291d1529..09d9fc33c84c092552b1523a4705b1c0a53b42b9 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -130,11 +130,15 @@ class SystemBubbleWrapper {
 SystemTray::SystemTray(StatusAreaWidget* status_area_widget)
     : TrayBackgroundView(status_area_widget),
       items_(),
+      detailed_item_(nullptr),
       default_bubble_height_(0),
       hide_notifications_(false),
       full_system_tray_menu_(false),
-      tray_accessibility_(NULL),
-      tray_date_(NULL) {
+      tray_accessibility_(nullptr),
+      tray_cast_(nullptr),
+      tray_date_(nullptr),
+      screen_capture_tray_item_(nullptr),
+      screen_share_tray_item_(nullptr) {

[stevenjb, 2016/03/30 00:47:57]

Bleh. These probably shouldn't exist, but something for another day. Thanks for
cleaning this up.

   SetContentsBackground();
 }