[Extensions] Start: real-time stream tab to the activity log page (1/2)

This CL adds a new tab in the activity log page that will be used to
display a real-time stream of extension activities.

Screenshots: https://imgur.com/a/MbqY2SR

Bug: 832354
Change-Id: I80a296215ad8b874ef977f8cff48a6733790527b
Reviewed-on: https://chromium-review.googlesource.com/c/1439969
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631815}
15 files changed