Oilpan: Do not add non-free headers to free list in ThreadHeap::assertEmpty.

It is possible that a thread is involved to GC after ThreadHeap::
assertEmpty before ThreadState destruction.

BUG=388267
R=ager@chromium.org, haraken@chromium.org

Review URL: https://codereview.chromium.org/372213002

git-svn-id: svn://svn.chromium.org/blink/trunk@177723 bbb929c8-8fbe-4397-9dbb-9b2b20218538