IDL: add overload counters in spec algorithm

Followup, adds the counter code for overloads.

TBR=haraken
BUG=293561

Review URL: https://codereview.chromium.org/275063003

git-svn-id: svn://svn.chromium.org/blink/trunk@173942 bbb929c8-8fbe-4397-9dbb-9b2b20218538