The {force_promotion} flag causes the scavenger to move an object to the
old generation instead of to the other semi-space. We use the flag to force
the promotion of objects which are referenced by code objects.

R=ulan@chromium.org

Review-Url: https://codereview.chromium.org/2002013002
Cr-Commit-Position: refs/heads/master@{#36443}