mash: Migrate tray date view to //ash/common types

ash/system/date/date_view.h

Index: ash/system/date/date_view.h
diff --git a/ash/system/date/date_view.h b/ash/system/date/date_view.h
index ac8beffe106a77eee62dde729946276739651d19..5a1a075a928aa28a4784452d5d35886969856091 100644
--- a/ash/system/date/date_view.h
+++ b/ash/system/date/date_view.h
@@ -15,6 +15,10 @@
 #include "base/timer/timer.h"
 #include "ui/views/view.h"
 
+namespace base {
+class Time;
+}
+
 namespace views {
 class Label;
 }