Tightening visibility of video_toolbox_cc.

This CL aligns the visibility to the standard decided in:
https://codereview.webrtc.org/3001623003.

BUG=webrtc:7743
NOTRY=True

Review-Url: https://codereview.webrtc.org/3010123002
Cr-Original-Commit-Position: refs/heads/master@{#19659}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6ff8f96baf339475fca80643bcd9d41e56f9fc4e
1 file changed