IDL compiler: [CustomConstructor] arguments and overloading

This finishes [CustomConstructor];
all we need to do is compute interface length correctly.

Updated docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-CustomConstructor-i-

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/100683004

git-svn-id: svn://svn.chromium.org/blink/trunk@163986 bbb929c8-8fbe-4397-9dbb-9b2b20218538