Remove Dictionary::getOwnPropertyNames()

This is a follow-up CL of https://codereview.chromium.org/661453004/.

I don't see why the call sites of getOwnPropertyNames() can't use
getPropertyNames() instead. The reason why they need property names
is that we don't support OpenEndedDictionary and IDL dictionary
which contains an union type at this point. In both cases we should
be able to use getPropertyNames().

Review URL: https://codereview.chromium.org/715583008

git-svn-id: svn://svn.chromium.org/blink/trunk@185098 bbb929c8-8fbe-4397-9dbb-9b2b20218538