Replace NULL with nullptr or null in webrtc/base/.

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2718663005
Cr-Original-Commit-Position: refs/heads/master@{#16878}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 37f5ecfd8f3337475872c9242221471784894f1c
117 files changed