Remove user-defined move constructor and assignment operator from WTF::String
Remove user-defined move constructor and assignment operator from WTF::String

WTF::String can rely on implicit move constructor and assignment
operator as WTF::RefPtr class has them defined now (r177066).

Review URL: https://codereview.chromium.org/367343003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@178541 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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