Update some uses of char16 to use the base:: namespace.

ash/system/user/tray_user.cc

Index: ash/system/user/tray_user.cc
diff --git a/ash/system/user/tray_user.cc b/ash/system/user/tray_user.cc
index 3d9e3ae78f86e27b7c6dd63a191f2f57e3273645..5159121203c2004bce926ad9f4ac9852663587df 100644
--- a/ash/system/user/tray_user.cc
+++ b/ash/system/user/tray_user.cc
@@ -89,7 +89,7 @@ const SkColor kBorderColor = 0xffdcdcdc;
 
 // The invisible word joiner character, used as a marker to indicate the start
 // and end of the user's display name in the public account user card's text.
-const char16 kDisplayNameMark[] = { 0x2060, 0 };
+const base::char16 kDisplayNameMark[] = { 0x2060, 0 };
 
 const int kPublicAccountLogoutButtonBorderImagesNormal[] = {
     IDR_AURA_TRAY_POPUP_PUBLIC_ACCOUNT_LOGOUT_BUTTON_BORDER,