Current use of background page and event page is confusing.
This document updates the terminology to Background Scripts
with an option to use it as a page in the manifest.
All background scripts should be persistent false,
with little to no exceptions.
This update had a huge focus on how to create effective
non-persistent background scripts.

Change-Id: I985caa739104d3608157c5e39119f9e27d57442e
Reviewed-on: https://chromium-review.googlesource.com/677735
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552873}