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: svn://svn.chromium.org/blink/trunk@181321 bbb929c8-8fbe-4397-9dbb-9b2b20218538