- Avoid calls into ToPositiveInteger for valid cases of 'undefined' arguments.
  (Otherwise it calls into runtime).
 - Reduce the checks performed in case offset for TypedArrayFromArrayBuffer
  constructor is called with no offset argument.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/59763010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17567 ce2b1a6d-e550-0410-aec6-3dcde31c8c00