This removes an overly complex predicate from the IsFlushable check
within the marking visitor. By now all JSFunction objects reference a
valid Context object, also builtin functions can be recognized without
looking at the JSFunction object.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1482363002

Cr-Commit-Position: refs/heads/master@{#32412}