GitLab

Make the chrome://extensions page use chrome.developerPrivate API to
select a file path for a crx and to perform the pack job itself.
Also add tests for the api function, and convert it to a
UIThreadExtensionFunction.

BUG=461039

Review URL: https://codereview.chromium.org/958803004

Cr-Commit-Position: refs/heads/master@{#318754}