Remove the user-defined empty WTF::String destructor

Remove the user-defined empty WTF::String destructor:
- It prevents from generating implicitly-declared move/copy
constructors (when C++11 features are enabled).
- Its removal decreases content_shell binary size by 8096 bytes
(linux release build) and slightly simplifies the WTF::String
class declaration.

Review URL: https://codereview.chromium.org/402193002

git-svn-id: svn://svn.chromium.org/blink/trunk@179209 bbb929c8-8fbe-4397-9dbb-9b2b20218538