Enhance SafeStackFrameIterator to avoid triggering assertions in debug mode.

Enhance SafeStackFrameIterator to avoid triggering assertions in debug mode.

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

Committed: http://code.google.com/p/v8/source/detail?r=5467

[Søren Thygesen Gjesse, 2010-09-16 06:49:15]

LGTM