IDL compiler: [Custom=PropertyEnumerator]

The meat of the CL is simple (just add another 2 boolean flags),
but there's some reorganization:
* Explicitly distinguish [Custom=PropertyEnumerator] vs.  [Custom=PropertyQuery]
(currently both lumped together, which makes the treatment of query weird)
* Fix order, so consistently
getter, setter, query, deleter, enumerator
(Matching V8 api.cc SetNamedPropertyHandler)

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/149993003

git-svn-id: svn://svn.chromium.org/blink/trunk@166122 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove superfluous isNull checks

The !isEmpty check already implies !isNull.

Review URL: https://codereview.chromium.org/149873002

git-svn-id: svn://svn.chromium.org/blink/trunk@166123 bbb929c8-8fbe-4397-9dbb-9b2b20218538