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-Commit-Position: refs/heads/master@{#19659}

webrtc/sdk/BUILD.gn

@@ -701,7 +701,7 @@ if (is_ios || is_mac) {
     }
 
     rtc_static_library("video_toolbox_cc") {
-      visibility = [ ":*" ]
+      visibility = [ ":videotoolbox_objc" ]
       sources = [
         "objc/Framework/Classes/VideoToolbox/helpers.cc",
         "objc/Framework/Classes/VideoToolbox/helpers.h",