Seeing occasional build breaks when building hls_builder, e.g.
https://travis-ci.org/google/shaka-packager/jobs/143445907
Although hls_builder is dependent on widevine_pssh_data_proto
indirectly (through media_base), it does not work all the time.
Looks like the proto has to be a direct dependency.
Change-Id: Id4d9d0b9e669df0967cc767132ef29717243a722