Replace Dictionary(Isolate*) constructor with a factory method Dictionary::createEmpty().

For better readability, based on a review comment.

Review URL: https://codereview.chromium.org/375013002

git-svn-id: svn://svn.chromium.org/blink/trunk@177711 bbb929c8-8fbe-4397-9dbb-9b2b20218538