IDL compiler: [Default] arguments

V. widely used.
Long-term we should instead use the Web IDL standard:
optional type identifier = value
instead of
[Default=value] optional type identifier
...which is much cleaner and clearer,
but that requires parser support and isn't worth doing in Perl.

Docs:
http://www.w3.org/TR/WebIDL/#dfn-optional-argument-default-value
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-Default-p-

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/47023015

git-svn-id: svn://svn.chromium.org/blink/trunk@161026 bbb929c8-8fbe-4397-9dbb-9b2b20218538