IDL compiler: put constructor contents together

Minor refactoring: in generate_interface, it's clearer to put the
constructor contents together, which helps group the code more
clear IMO.

Review URL: https://codereview.chromium.org/148023004

git-svn-id: svn://svn.chromium.org/blink/trunk@165844 bbb929c8-8fbe-4397-9dbb-9b2b20218538