More compact way to disallow implicit WTF::String conversion to boolean

Instead of introducing an ambiguousness and an explanation comment for it,
we could do the same thing we do in 'WTF_MAKE_NONCOPYABLE' macro - simply
declare the conversion operator privately.

Review URL: https://codereview.chromium.org/150573012

git-svn-id: svn://svn.chromium.org/blink/trunk@167172 bbb929c8-8fbe-4397-9dbb-9b2b20218538