rtc::FunctionView improvements: accept function pointers and nullptr

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2375023004
Cr-Original-Commit-Position: refs/heads/master@{#14483}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 607d9d7f604f7d8cc4a427de38db06eb674482b9
2 files changed