Prefix string16 with base:: in ui/.

ui/message_center/cocoa/settings_controller.mm

Index: ui/message_center/cocoa/settings_controller.mm
diff --git a/ui/message_center/cocoa/settings_controller.mm b/ui/message_center/cocoa/settings_controller.mm
index 82f2c46188fe008b6e3c3476bab78d8d4aa7028e..1f8e2045171916b1d3c0b48da0ff2894efef5c51 100644
--- a/ui/message_center/cocoa/settings_controller.mm
+++ b/ui/message_center/cocoa/settings_controller.mm
@@ -192,11 +192,12 @@ void NotifierSettingsObserverMac::NotifierGroupChanged() {
     [groupDropDownButton_ setTarget:self];
     // Add a dummy item for pull-down.
     [groupDropDownButton_ addItemWithTitle:@""];
-    string16 title;
+    base::string16 title;
     for (size_t i = 0; i < groupCount; ++i) {
       const message_center::NotifierGroup& group =
           provider_->GetNotifierGroupAt(i);
-      string16 item = group.login_info.empty() ? group.name : group.login_info;
+      base::string16 item =
+          group.login_info.empty() ? group.name : group.login_info;
       [groupDropDownButton_ addItemWithTitle:base::SysUTF16ToNSString(item)];
       if (provider_->IsNotifierGroupActiveAt(i)) {
         title = item;