ash: Initialize some pointers to null in SystemTray constructor

They eventually get valid pointers assigned, but this makes the code clearer
and prevents spurious warnings from editors (like Eclipse) that detect
constructors with uninitialized data.

BUG=none
TEST=existing ash tests

Review URL: https://codereview.chromium.org/1838343003

Cr-Commit-Position: refs/heads/master@{#383891}

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) {
   SetContentsBackground();
 }