Sampler can retrieve current simulator for profiled isolate from its ThreadLocalTop without calls to Isolate::FindPerThreadDataForThread which sometimes leads to acquring same mutex second time.

BUG=v8:2874
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/25053002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17047 ce2b1a6d-e550-0410-aec6-3dcde31c8c00