Add an event observer for logged Background Service events.

Adds an event to inform the DevTools frontend about new Background Service
events. Sends all existing (logged) events when enabled, and updates the
frontend with new events as they come along.

Bug: 927726
Change-Id: Id4ee17df8385179207d461ebfa33e031a0b79cac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1493051
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639461}
8 files changed