DASH manifests can now contain non-null but incomplete
DRM init data. Hence using the manifest init data when
non-null is not always the correct thing to do. This
change merges the sample and manifest formats (which
correctly merges the DRM init data) and then uses the
result.
Issue: #3630
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=180787784
