Enable WTF::HashSet<OwnPtr<> >

Allow HashSet to have OwnPtr elements, this helps in
cases when the container needs to own its elements.
The client does not need to care about explicit calling
'deleteAllValues' funcion (it can be deprecated now).

The patch contains unit tests, and examples of real
using HashSet<OwnPtr<> > in XPathParser.

Review URL: https://codereview.chromium.org/252633005

git-svn-id: svn://svn.chromium.org/blink/trunk@172738 bbb929c8-8fbe-4397-9dbb-9b2b20218538