It broke the Mac build.

Sampler::SampleStack() can't use Isolate::Current() when called from
the sampler thread (from SampleContext). We have to pass the isolate
pointer.

TBR=mikhail.naganov@gmail.com

Review URL: http://codereview.chromium.org/6791014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7485 ce2b1a6d-e550-0410-aec6-3dcde31c8c00