High-level tracking bug for Web Share on Desktop.

This covers:
- navigator.share API (Web Share)
- share_target manifest attribute (Web Share Target)
- "Share" menu item to share to a Web share target (Issue 738273)

Status: The first two are already implemented behind the --enable-experimental-web-platform-features flag (in stable channel). See https://wicg.github.io/web-share-target/demos/sharetarget.html for instructions.

The share target picker needs UX treatment.