Add explicit base namespace to string16 users.

chrome/common/extensions/manifest_handler_helpers.cc

Index: chrome/common/extensions/manifest_handler_helpers.cc
diff --git a/chrome/common/extensions/manifest_handler_helpers.cc b/chrome/common/extensions/manifest_handler_helpers.cc
index 1e66fe4bba87bc4d0572fcec036be945905be7f6..f46fd3130a273ecf1b58a878bbef2ae30f12a373 100644
--- a/chrome/common/extensions/manifest_handler_helpers.cc
+++ b/chrome/common/extensions/manifest_handler_helpers.cc
@@ -35,7 +35,7 @@ bool LoadIconsFromDictionary(const base::DictionaryValue* icons_value,
                              const int* icon_sizes,
                              size_t num_icon_sizes,
                              ExtensionIconSet* icons,
-                             string16* error) {
+                             base::string16* error) {
   DCHECK(icons);
   for (size_t i = 0; i < num_icon_sizes; ++i) {
     std::string key = base::IntToString(icon_sizes[i]);