Reland^3 r22105 "Remove static initializer from isolate"

This time:
When accessing the isolate from the sampler signal handler, don't try
to take the global lock. A sampler should only be active if there is
already an isolate on that thread, so we don't need to check whether
the TLS key is already created.

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/378513002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22235 ce2b1a6d-e550-0410-aec6-3dcde31c8c00