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:7835, webrtc:7841 Review-Url: https://codereview.webrtc.org/3001483002 Cr-Commit-Position: refs/heads/master@{#19427}
BUG=webrtc:7835, webrtc:7841 Review-Url: https://codereview.webrtc.org/2996693002 Cr-Commit-Position: refs/heads/master@{#19409}