Convert string16 to base::string16 in content.

content/browser/accessibility/accessibility_tree_formatter_mac.mm

Index: content/browser/accessibility/accessibility_tree_formatter_mac.mm
diff --git a/content/browser/accessibility/accessibility_tree_formatter_mac.mm b/content/browser/accessibility/accessibility_tree_formatter_mac.mm
index 58881b4af4b18beb7bd0388ee7eff9102c897668..021b5fdc419ce644b66fb68b1075f4468c58c211 100644
--- a/content/browser/accessibility/accessibility_tree_formatter_mac.mm
+++ b/content/browser/accessibility/accessibility_tree_formatter_mac.mm
@@ -161,9 +161,10 @@ void AccessibilityTreeFormatter::AddProperties(const BrowserAccessibility& node,
   dict->Set(kSizeDictAttr, PopulateSize(cocoa_node).release());
 }
 
-string16 AccessibilityTreeFormatter::ToString(const base::DictionaryValue& dict,
-                                              const string16& indent) {
-  string16 line;
+base::string16 AccessibilityTreeFormatter::ToString(
+    const base::DictionaryValue& dict,
+    const base::string16& indent) {
+  base::string16 line;
   NSArray* defaultAttributes =
       [NSArray arrayWithObjects:NSAccessibilityTitleAttribute,
                                 NSAccessibilityValueAttribute,