IDL compiler: [NotEnumerable]

Question: the PropertyAttribute list always includes 'v8::None',
but this looks redundant (it's 0, so 0 | is a nop):
v8::None | v8::DontEnum == v8::DontEnum
Instead, shouldn't we only have v8::None if there are no other
PropertyAttribute values?
(Easy change to Perl CG.)

BUG=239771

Review URL: https://codereview.chromium.org/26954004

git-svn-id: svn://svn.chromium.org/blink/trunk@159665 bbb929c8-8fbe-4397-9dbb-9b2b20218538