Mojo cpp bindings: fix |num_fields| in struct header of *Params_Data.

BUG=None
TEST=None

Review URL: https://codereview.chromium.org/291773002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271352 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -32,7 +32,7 @@ class {{class_name}} {
  private:
   {{class_name}}() {
     _header_.num_bytes = sizeof(*this);
-    _header_.num_fields = 3;
+    _header_.num_fields = {{struct.packed.packed_fields|length}};
   }
 
   mojo::internal::StructHeader _header_;