Add support for mojo::TypeConverter to control how convenient conversion should be.

mojo/common/common_type_converters.h

Index: mojo/common/common_type_converters.h
diff --git a/mojo/common/common_type_converters.h b/mojo/common/common_type_converters.h
index c8a1ddde386ee77b9cfed7fe378ce8bed18d5169..a43d18b5fe49a7b8c6a7f8a3fdb8eb6655478224 100644
--- a/mojo/common/common_type_converters.h
+++ b/mojo/common/common_type_converters.h
@@ -18,6 +18,8 @@ class MOJO_COMMON_EXPORT TypeConverter<String, base::StringPiece> {
  public:
   static String ConvertFrom(const base::StringPiece& input, Buffer* buf);
   static base::StringPiece ConvertTo(const String& input);
+
+  MOJO_ALLOW_IMPLICIT_TYPE_CONVERSION();
 };
 
 template <>
@@ -25,6 +27,8 @@ class MOJO_COMMON_EXPORT TypeConverter<String, base::string16> {
  public:
   static String ConvertFrom(const base::string16& input, Buffer* buf);
   static base::string16 ConvertTo(const String& input);
+
+  MOJO_ALLOW_IMPLICIT_TYPE_CONVERSION();
 };
 
 }  // namespace mojo