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

mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl

Index: mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl
index 7c429c01b4661424d45eda449d21072d815e8d03..df17833b17dccd6f454831352fc1022045c9ea0c 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl
@@ -13,17 +13,20 @@ class {{struct.name}} {
 
   template <typename U>
   {{struct.name}}(const U& u, mojo::Buffer* buf = mojo::Buffer::current()) {
+    MOJO_INTERNAL_CHECK_ALLOW_DIRECT_TYPE_CONVERSION({{struct.name}}, U);
     *this = mojo::TypeConverter<{{struct.name}},U>::ConvertFrom(u, buf);
   }
 
   template <typename U>
   {{struct.name}}& operator=(const U& u) {
+    MOJO_INTERNAL_CHECK_ALLOW_DIRECT_TYPE_CONVERSION({{struct.name}}, U);
     *this = mojo::TypeConverter<{{struct.name}},U>::ConvertFrom(u, mojo::Buffer::current());
     return *this;
   }
 
   template <typename U>
   operator U() const {
+    MOJO_INTERNAL_CHECK_ALLOW_DIRECT_TYPE_CONVERSION({{struct.name}}, U);
     return To<U>();
   }
 
@@ -32,6 +35,11 @@ class {{struct.name}} {
     return mojo::TypeConverter<{{struct.name}},U>::ConvertTo(*this);
   }
 
+  template <typename U>
+  static {{struct.name}} From(const U& u, mojo::Buffer* buf = mojo::Buffer::current()) {
+    return mojo::TypeConverter<{{struct.name}},U>::ConvertFrom(u, buf);
+  }
+
   bool is_null() const { return !data_; }
 
 {#--- Getters #}