The concept of class was mostly removed from ES6, but we still use the
class of objects to distinguish object kinds in our builtins.  So update
this to be in sync with IsCallable (thereby getting rid of the previous
instance type based tests for callable things completely).

R=jarin@chromium.org, jkummerow@chromium.org

Review URL: https://codereview.chromium.org/1307943013

Cr-Commit-Position: refs/heads/master@{#30566}