GitLab

Previously, chrome url overrides were added and removed at extension load and
unload. They should instead be added at install time, updated when the extension
is unloaded, and removed when the extension is uninstalled. Change the pref from
a string (of the url) to a dictionary with { entry: <url>, active: <bool> } to
allow for inactive, but retained, entries.

Also add regression tests.

BUG=571429

Review URL: https://codereview.chromium.org/1543923002

Cr-Commit-Position: refs/heads/master@{#367137}