Prefix string16 with base:: in ui/.

ui/views/controls/button/image_button.cc

Index: ui/views/controls/button/image_button.cc
diff --git a/ui/views/controls/button/image_button.cc b/ui/views/controls/button/image_button.cc
index 6a842346d2c462bc015ae19ff25d13591cb80820..fc804e706c397cc4220cd6ef691d1696f7945e00 100644
--- a/ui/views/controls/button/image_button.cc
+++ b/ui/views/controls/button/image_button.cc
@@ -220,7 +220,7 @@ void ToggleImageButton::SetToggledImage(ButtonState state,
   }
 }
 
-void ToggleImageButton::SetToggledTooltipText(const string16& tooltip) {
+void ToggleImageButton::SetToggledTooltipText(const base::string16& tooltip) {
   toggled_tooltip_text_ = tooltip;
 }
 
@@ -249,7 +249,7 @@ void ToggleImageButton::SetImage(ButtonState state,
 // ToggleImageButton, View overrides:
 
 bool ToggleImageButton::GetTooltipText(const gfx::Point& p,
-                                       string16* tooltip) const {
+                                       base::string16* tooltip) const {
   if (!toggled_ || toggled_tooltip_text_.empty())
     return Button::GetTooltipText(p, tooltip);