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

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

 class {{struct.name}} {
  public:
   typedef internal::{{struct.name}}_Data Data;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
 {%    macro enum_def() %}{% include "enum_declaration.tmpl" %}{% endmacro %}
   {{enum_def()|indent(2)}}
 {%- endfor %}
 
   {{struct.name}}() : data_(NULL) {
   }
 
   template <typename U>
   {{struct.name}}(const U& u, mojo::Buffer* buf = mojo::Buffer::current()) {
+    MOJO_INTERNAL_CHECK_ALLOW_IMPLICIT_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_IMPLICIT_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_IMPLICIT_TYPE_CONVERSION({{struct.name}}, U);
     return To<U>();
   }
 
   template <typename U>
   U To() const {
     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 #}
 {%  for packed_field in struct.packed.packed_fields %}
 {%-   set type = packed_field.field.kind|cpp_wrapper_type %}
 {%-   set name = packed_field.field.name %}
 {%-   if packed_field.field.kind|is_object_kind %}
   const {{type}} {{name}}() const { {#
     #}return mojo::internal::Wrap(data_->{{name}}()); }
 {%-   elif packed_field.field.kind|is_handle_kind %}
@@ skipping 33 matching lines @@
   };
 
  private:
   friend class mojo::internal::WrapperHelper<{{struct.name}}>;
 
   struct Wrap {};
   {{struct.name}}(Wrap, const Data* data) : data_(data) {}
 
   const Data* data_;
 };