GitLab

- Web Share spec has moved from WICG to W3C.
- Updated some Web Share Target URLs to point to the WICG spec, not the
  explainer.
- Updated some Web Share Target URLs to point to the WICG spec, not a
  draft on the pr-preview site.
- Removed some notes that say Web Share Target is part of an unlaunched
  experiment.

Change-Id: I48132388192d773d962d13bc9361778ddeb66e4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742258Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685457}