Adds support for WebM to the CDM. Decryption remains untouched,
however the initialization data is passed differently for WebM.
The previous version of this change broke playback for certain
apps that were being allowed to pass invalid MIME types before
this change was made. This version maintains backwards-compatiblity
for these apps for now by rewriting their MIME types as "video/mp4".
Merge of https://widevine-internal-review.googlesource.com/9225/
and https://widevine-internal-review.googlesource.com/9611/ from
the Widevine cdm repo.
Bug: 10638562
Change-Id: Ib37e838d08363f07b34b3a2e79a3f80a1f43e9ad
702aadf853