Adds support for WebM to the CDM. Decryption remains untouched,
however the initialization data is passed differently for WebM.
Merge of https://widevine-internal-review.googlesource.com/#/c/9225/
from the widevine cdm repo.
Bug: 10638562
Change-Id: I7b8cf4888fa408af77cee103f768f5a7c8ffdc7e