Remove String(RefPtr<StringImpl>) constructor

The String(RefPtr<StringImpl>) constructor should be removed for the following reasons:
1) It almost does not have clients, so it is unneeded.
2) RefPtr should never be passed by value

Review URL: https://chromiumcodereview.appspot.com/23872003

git-svn-id: svn://svn.chromium.org/blink/trunk@157321 bbb929c8-8fbe-4397-9dbb-9b2b20218538