Step 10 of EME's "Get Supported Capabilities for Audio/Video Type" algorithm
says we can assume default codecs only if a container normatively implies a
specific set of codec and codec constraints. Our code assumes that WebM implies
Vorbis/VP8 and MP4 implies AAC/H.264, but those aren't actually normatively
required by either of these containers' specifications. So we shouldn't assume
these containers imply those codecs.
MozReview-Commit-ID: G9TDOmrjhpp
6e7cbe7323