GitLab

Currently only shares to a demo Share Target App hosted at
https://wicg.github.io/web-share-target/demos/sharetarget.html;
follow-up CL will have more share options.

BUG=668389

Review-Url: https://codereview.chromium.org/2564483003
Cr-Commit-Position: refs/heads/master@{#443510}