Remove callback_cs_ in ViEEncoder. Instead make callbacks const and set on construction. BUG=webrtc:1695 R=philipel@webrtc.org Review URL: https://codereview.webrtc.org/1354143004 . Cr-Original-Commit-Position: refs/heads/master@{#10017} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: 7083e119e8f39d2ec9e504461c1bb6e0bc6be5ff