Avoid TLS accesses in Object::Lookup and Object::GetPrototype.

Both methods were among the top causes for TLS accesses.

BUG=v8:2487

Review URL: https://codereview.chromium.org/12319144

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13759 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Increase acceptable boot up memory size in tests for parallel sweeper threads.

BUG=

Review URL: https://codereview.chromium.org/12310168

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13760 ce2b1a6d-e550-0410-aec6-3dcde31c8c00