Remove kProtectionKey* and VCMKeyRequestMode.

Enforces previous kProtectionKeyOnLoss as the permanent method which was
the only one used in use. This simplifies SetVideoProtection and
transition over to SetReceiverRobustnessMode.

BUG=webrtc:1596
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1244753002

Cr-Commit-Position: refs/heads/master@{#9641}
5 files changed