Move StackTracer to sampler.h

Apart from tests Sampler is the only client of StackTracer so it is logical to move it into sampler.h

BUG=None

Review URL: https://codereview.chromium.org/14208012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14298 ce2b1a6d-e550-0410-aec6-3dcde31c8c00