The callbacks are per isolate, so we shouldn't get the isolate implicitly
from TLS. Also, we shouldn't allow calls to these methods prior to
initializing the respective isolate (and silently ignore them).

Esp. add a per-isolate API to set the stats counter callbacks and
make it possible to set the stats counter callback after the isolate
was touched.

Embedders should use e.g. isolate->SetCounterFunction(callback) instead
of v8::V8::SetCounterFunction(callback).

BUG=none
R=svenpanne@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/365153002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22223 ce2b1a6d-e550-0410-aec6-3dcde31c8c00