Add PrintTo for string16

base/string16.h

Index: base/string16.h
diff --git a/base/string16.h b/base/string16.h
index 4c7b21f8a0eb76e34a0d4887dd19ca01c944b284..b6dcd0313da6f2cd364df3c63ccbe954da750b18 100644
--- a/base/string16.h
+++ b/base/string16.h
@@ -171,6 +171,9 @@ typedef std::basic_string<char16, base::string16_char_traits> string16;
 namespace base {
 BASE_EXPORT extern std::ostream& operator<<(std::ostream& out,
                                             const string16& str);
+
+// This is required by googletest to print a readable output on test failures.
+BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
 }
 
 #endif  // WCHAR_T_IS_UTF32