This fixes flaky crashes in gc-stress bot:
> Fatal error in ../src/incremental-marking.cc, line 84
> CHECK(obj->IsHeapObject()) failed

BUG=353551
LOG=N
TEST=test/mjsunit/regress/regress-353551.js
R=m.m.capewell@googlemail.com

Review URL: https://codereview.chromium.org/204453002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20098 ce2b1a6d-e550-0410-aec6-3dcde31c8c00