Wait for sweeper threads when verifying the heap.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/396793005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22475 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Always sweep precisely.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/394023007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22476 ce2b1a6d-e550-0410-aec6-3dcde31c8c00