Revert of Remove user-defined move constructor and assignment operator from WTF::String (https://codereview.chromium.org/367343003/)

Reason for revert:
This can be applied only when user-defined dtor is removed.

Original issue's description:
> 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).
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178541

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/402163002

git-svn-id: svn://svn.chromium.org/blink/trunk@178561 bbb929c8-8fbe-4397-9dbb-9b2b20218538