Reland r178541. Remove user-defined move constructor and assignment operator from WTF::String.
Reland r178541. Remove user-defined move constructor and assignment operator from WTF::String.

It is possible as WTF::String does not have user-defined destructor any more (r179209).

BUG=410302

Review URL: https://codereview.chromium.org/486293002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@181321 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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