We use Optional in our public API, so its header should be in webrtc/api/. BUG=webrtc:8205 Review-Url: https://codereview.webrtc.org/3011943002 Cr-Commit-Position: refs/heads/master@{#19693}
BUG=webrtc:7832, webrtc:7838 Review-Url: https://codereview.webrtc.org/2962653002 Cr-Commit-Position: refs/heads/master@{#19190}