IDL compiler: consolidate [Custom*] interface to [Custom=*]

Cleanup, simplifying and clarifying extended attributes.
(No changes to generated bindings.)

This merges the 4 basic [Custom*] extended attributes on interfaces:
[CustomLegacyCall] (renamed to LegacyCallAsFunction)
[CustomToV8]
[CustomVisitDOMWrapper]
[CustomWrap]
=> [Custom=LegacyCallAsFunction|ToV8|VisitDOMWrapper|Wrap]

[CustomConstructor] is different, as it takes a list of parameters,
and can be overloaded (grammatically it's a ExtendedAttributeArgList).

Also minor Perl fix to extended attribute value list parsing.

Docs updated:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-Custom-i-m-a-

Review URL: https://codereview.chromium.org/107843002

git-svn-id: svn://svn.chromium.org/blink/trunk@163392 bbb929c8-8fbe-4397-9dbb-9b2b20218538