[Extensions] Small UI changes for activity log

Summary of changes:
 - Move the clear activities button under the history/stream tabs
 - Add extension name and icon to activity log page
 - Add headings above activity log items

Refactoring of styles may be done in a follow up CL

Screenshots included in:

Bug: 932768
Change-Id: I249cee13e1b5c7e4bd319b5b788a0c05718ba8da
Reviewed-on: https://chromium-review.googlesource.com/c/1483738
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636668}
21 files changed