Add Badging experimental web API.

This CL introduces the badging api. The badging API will eventually
allow developers to set and clear badges for installed PWAs.

Intent to implement:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/Fw764MVF5nI

Explainer:
https://github.com/WICG/badging/blob/master/explainer.md

Bug: 719176
Change-Id: I68aa5a9310469bd3a38b1029f2141be335121b06
Reviewed-on: https://chromium-review.googlesource.com/1166603
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595587}
25 files changed