Cleaning up usage of lookup results.

Cleaning up usage of lookup results.

- Ensure that IsFound() is only used when not in combination with other
  checks. To do so, the default type is NONEXISTENT rather than NORMAL;
  and NotFound() also resets the type to NONEXISTENT.
- Use test methods rather than .type() == A_PROPERTY_TYPE.

Review URL: https://chromiumcodereview.appspot.com/10626004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11899 ce2b1a6d-e550-0410-aec6-3dcde31c8c00