This is result of running 2to3 without dict fix (as it seems
unnecessary for most of our cases).
In Python 3 {}.values() returns a view that is not indexable. This
commit uses list() on that to make AddHandler.should_callback compatible
with Python 3.
Change-Id: I354597f43d43630f9fb875dd8c9ab741c35af723
7ed6e86744