Added system thread manager class.

BUG=

Review URL: https://codereview.chromium.org/12207207

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13757 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Shrinking of PagedSpace is done only by sweeper.

BUG=

Review URL: https://codereview.chromium.org/12313131

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13758 ce2b1a6d-e550-0410-aec6-3dcde31c8c00