Adding a namespace comment

This was pointed out in https://codereview.webrtc.org/1772583002/.

BUG=webrtc:5607 NOTRY=true
TBR=kwiberg@webrtc.org

Review URL: https://codereview.webrtc.org/1774933002

Cr-Original-Commit-Position: refs/heads/master@{#11908}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bc89de3bca89b24bac3b6edae2a1bdc609f16aa6
1 file changed