Use C++11 enum class in Blink

Use C++11 enum class in Blink and get rid of its home-brewed analogue.

Review URL: https://codereview.chromium.org/650013009

git-svn-id: svn://svn.chromium.org/blink/trunk@184506 bbb929c8-8fbe-4397-9dbb-9b2b20218538