Add output formatters for more mojom types.

mojo/public/cpp/bindings/string.h

Index: mojo/public/cpp/bindings/string.h
diff --git a/mojo/public/cpp/bindings/string.h b/mojo/public/cpp/bindings/string.h
index 4cf863192b64e86595106df5e65261eeb182245f..7f2ca313aacd997d57e971abddc2111e365886d5 100644
--- a/mojo/public/cpp/bindings/string.h
+++ b/mojo/public/cpp/bindings/string.h
@@ -5,6 +5,7 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_STRING_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRING_H_
 
+#include <iosfwd>
 #include <string>
 
 #include "mojo/public/cpp/bindings/lib/array_internal.h"
@@ -122,6 +123,8 @@ inline bool operator!=(const String& a, const char* b) {
   return !(a == b);
 }
 
+// TODO(jeffbrown): Decide whether this should print a sentinel value
+// such as "<null>" when formatting null strings.
 inline std::ostream& operator<<(std::ostream& out, const String& s) {
   return out << s.get();
 }