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..4fdd04f741c56c8228c4e8f048500f6d295bebcd 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_DIRECT_TYPE_CONVERSION();

[darin (slow to review), 2014/04/23 18:01:01]

Why don't you like using the term "implicit" here? Isn't that the opposite of
explicit, and in C++ the explicit keyword is what you use to disallow implicit
constructors. Seems like implicit has the right meaning here, no?

MOJO_ALLOW_IMPLICIT_TYPE_CONVERSION()

[yzshen1, 2014/04/23 18:20:57]

Because it sounds like we allow "explicit conversion" without the macro. And
"explicit conversion" makes me think of things such as "A a =
static_cast<A>(b);", which we don't support without the macro.

(But I don't have a very strong preference. I am fine if you think that is
better.)

 };
 
 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_DIRECT_TYPE_CONVERSION();
 };
 
 }  // namespace mojo