Prepare for incremental sweep by making the FreeList independent of the ThreadHeap

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/711053002

git-svn-id: svn://svn.chromium.org/blink/trunk@185099 bbb929c8-8fbe-4397-9dbb-9b2b20218538