Merge from Widevine repo of http://go/wvgerrit/96163
This CL just addresses some review comments from the big merge to
master. The header OEMCryptoCENC.h is now synced with the
document http://go/oemcrypto.
Test: unit tests
Bug: 148907684
Change-Id: Ic825126e0dd3d7e86eefab2c51b4abb5d57fb568