Remove ViEExternalCodec usage in VideoSendStream.

Replaces interface usage with direct calls on ViEEncoder removing a
layer of indirection.

BUG=1695
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49179004

Cr-Commit-Position: refs/heads/master@{#9095}
2 files changed