Overload resolution: ignore extra arguments

2.5 year old bug, now an easy fix.

Implement this step of the algorithm:
2. Initialize argcount to be min(maxarg, n).

This should be safe: it just allows looser calls (JS-style), and does not
change any existing correct calls.

R=haraken, jsbell
BUG=102925

Review URL: https://codereview.chromium.org/291563004

git-svn-id: svn://svn.chromium.org/blink/trunk@174181 bbb929c8-8fbe-4397-9dbb-9b2b20218538