When running profiling in debug mode, several assertions in frame
iterators that are undoubtedly useful when iterator is started from a
VM thread in a known "good" state, may fail when running over a stack
of a suspended VM thread. This patch makes SafeStackFrameIterator
to proactively check addresses and bail out from iteration early,
before an assertion will be triggered.

BUG=crbug/55565

Review URL: http://codereview.chromium.org/3436006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5467 ce2b1a6d-e550-0410-aec6-3dcde31c8c00