ui/gfx: Use base::string16 now that string16 was moved into base namespace.

ui/gfx/platform_font_ios.mm

Index: ui/gfx/platform_font_ios.mm
diff --git a/ui/gfx/platform_font_ios.mm b/ui/gfx/platform_font_ios.mm
index f0f559de51cb2eacbb66683782b60db5a9643738..cfc69cf4964bfa0b6573220e203300c672a22273 100644
--- a/ui/gfx/platform_font_ios.mm
+++ b/ui/gfx/platform_font_ios.mm
@@ -55,7 +55,7 @@ int PlatformFontIOS::GetAverageCharacterWidth() const {
   return average_width_;
 }
 
-int PlatformFontIOS::GetStringWidth(const string16& text) const {
+int PlatformFontIOS::GetStringWidth(const base::string16& text) const {
   NSString* ns_text = base::SysUTF16ToNSString(text);
   return [ns_text sizeWithFont:GetNativeFont()].width;
 }