This makes some tests in test-heap.cc more robust against flags, now
that top-level code can be optimized by the --always-opt flag.

R=hpayer@chromium.org
TEST=cctest/test-heap/TestInternalWeakLists

Review URL: https://codereview.chromium.org/1071063004

Cr-Commit-Position: refs/heads/master@{#27699}