rtc::Optional: Tell sanitizers that unset values aren't OK to access

This is a sample use of the other three sanitizer functions introduced
in https://codereview.webrtc.org/2293893002/.

BUG=chromium:617124

Review-Url: https://codereview.webrtc.org/2289383002
Cr-Original-Commit-Position: refs/heads/master@{#14157}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2b11fd2e4bcbd5240ee04110323309f1fa2d3251
4 files changed