Add filler at the new space top when forcing scavenge.

We only seem to force scavenge in our cctest test suite, so this is
expected to fix some flakiness in our tests, but it will not
improve stability of v8 itself.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/167423004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00