Fix issue introduced by cell iterator refactor.

The IsValid() method was changed to IsDoneWithTraversal(), reversing
the logic of the method. However, the implementations/usages weren't
reversed.

Change-Id: I825616564d79cacc81a688b5fd9b04c9c7a15d8d
Merge topic 'fix-celliterator-usage' into master

4ba6f4cc Fix issue introduced by cell iterator refactor.