Fix spacing style for anonymous special operations

Anonymous special operations should be spaced like this:
getter DOMString (unsigned long index);
...not like this:
getter DOMString(unsigned long index);
...b/c this looks like a function called "DOMString"!
(This follows spec.)

Fixed and documented:
https://sites.google.com/a/chromium.org/dev/blink/webidl#TOC-Style

TBR=haraken

Review URL: https://codereview.chromium.org/148153002

git-svn-id: svn://svn.chromium.org/blink/trunk@165843 bbb929c8-8fbe-4397-9dbb-9b2b20218538