Get rid of Isolate::safe_stack_iterator_counter

This change removes per-isolate counter of active SafeStackFrameIterators. The counter is used by stack frames implementations to avoid accessing pointers to heap objects when traversing stack for CPU profiler (so called "safe" mode). Each StackFrame instance is owned by single iterator and has a pointer to it so we can simply mark the iterator as "safe" or not and read the field in the stack frames instead of going into the isolate.

BUG=None
R=loislo@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/17585008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15317 ce2b1a6d-e550-0410-aec6-3dcde31c8c00