Replace NULL with nullptr in webrtc/video.

BUG=
R=danilchap@webrtc.org

Review URL: https://codereview.webrtc.org/1855433002 .

Cr-Original-Commit-Position: refs/heads/master@{#12218}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 74f6e9ea2329a56d8e202f70d3a5bc6d9a7527d9
19 files changed