IDL compiler: [NotEnumerable] special operations

Note that [NotEnumerable] is only used on the *getter*;
one nop use on setter+deleter, which I've removed.

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/150023002

git-svn-id: svn://svn.chromium.org/blink/trunk@166205 bbb929c8-8fbe-4397-9dbb-9b2b20218538