Use std::move in AtomicString
https://bugs.webkit.org/show_bug.cgi?id=115102

Reviewed by Anders Carlsson.

Use std::move in AtomicString instead of direct casting to rvalue references.

* wtf/text/AtomicString.h:
(AtomicString):
(WTF::AtomicString::AtomicString):
(WTF::AtomicString::operator=):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149112 268f45cc-cd09-0410-ab3c-d52691b4dbfc