Merge SafeStackTraceFrameIterator into SafeStackFrameIterator

SafeStackFrameIterator was used solely to implement SafeStackTraceFrameIterator. This CL simply merges them and updates usage of SafeStackTraceFrameIterator to use SafeStackFrameIterator (a bit shorter name).

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

Review URL: https://codereview.chromium.org/17579005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15305 ce2b1a6d-e550-0410-aec6-3dcde31c8c00