Implemented stub ShareService mojo service, for navigator.share.

For desktop Linux, Chrome OS and Windows only.

Only active behind the
--enable-experimental-web-platform-features flag.

BUG=668389
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2545323002
Cr-Commit-Position: refs/heads/master@{#438763}
7 files changed