Remove a race between sweepers and the free space skipping code (while iterating...

Remove a race between sweepers and the free space skipping code (while iterating new space pointers).

There has been a race between a sweeper setting the next pointer on
free list node and the main thread skipping free space during update of
new space pointers in the heap.

This change removes the free space skipping code.

R=hpayer@chromium.org
BUG=370551
LOG=N

Review URL: https://codereview.chromium.org/285733003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21306 ce2b1a6d-e550-0410-aec6-3dcde31c8c00