This fixes the materialization of JSFunction objects to not rely on a
context being available. The context has been cleared because it might
be de-materiallized itself.

R=bmeurer@chromium.org
TEST=mjsunit/compiler/escape-analysis-materialize
BUG=chromium:644245

Review-Url: https://codereview.chromium.org/2320983002
Cr-Commit-Position: refs/heads/master@{#39277}