IDL compiler: [Unforgeable]

Straightforward, same "Remove superfluous default" as with property attributes
(in Perl too), in this case v8::DEFAULT.
Docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-Unforgeable-m-a-

BUG=239771

Review URL: https://codereview.chromium.org/27476003

git-svn-id: svn://svn.chromium.org/blink/trunk@159843 bbb929c8-8fbe-4397-9dbb-9b2b20218538