Changed CalledAsConstructor to process the raw runtime stack directly,
rather than using a StackFrameIterator.  Using an interator turns out
to be quite expensive and the vast majority of the work done is not
relevant to deciding if we've been called as a constructor.

This speeds up getElementById by ~13%.

Review URL: http://codereview.chromium.org/160325

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2578 ce2b1a6d-e550-0410-aec6-3dcde31c8c00