Add move semantics to RefPtr
https://bugs.webkit.org/show_bug.cgi?id=115033

Reviewed by Anders Carlsson.

Add move constructors and move assignment operators to RefPtr when
COMPILER_SUPPORTS(CXX_RVALUE_REFERENCES).
This obviates unnecessary reffing/ureffing when RefPtr is created
or assigned from rvalue references.

* wtf/RefPtr.h:
(RefPtr):
(WTF::RefPtr::RefPtr):
(WTF::RefPtr::operator=):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149184 268f45cc-cd09-0410-ab3c-d52691b4dbfc