[Extensions] Fix table appearance in activity log view

Change appearance of the activity log to be more inline with other webui
based tables. This CL also adds the functionality for activity log items
to be expanded and collapsed if an item is associated with one or more
urls.

A placeholder delete icon is also added for this CL. The functionality
for deleting specific activity log items will be implemented in a future
CL.

How to test changes:

1) Run chrome locally with the flag "enable-extension-activity-logging"
i.e. "out/Default/chrome --enable-extension-activity-logging"

2) navigate to chrome://extensions (install extensions if needed)

3) click on "Details" for an extension to see the details page.

4) click on "View Activity log" on the details page for that extension.

2b) Alternatively, the activity log can be found at:
chrome://extensions/?activity=<extension_id>

Design mock: https://screenshot.googleplex.com/WAtvuqifSDM.png

Screenshots:
Before: https://imgur.com/a/ZUIBtE9

After: https://imgur.com/a/5CCeQf9
With arrows showing alignment specs: https://imgur.com/a/DwPu51k
Old (Patchset 1) https://imgur.com/a/LzPGB5d

Bug: 908595
Change-Id: I7d5f8aa5f5050bee929cdc4489f80114d7576d75
Reviewed-on: https://chromium-review.googlesource.com/c/1381375
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622193}
8 files changed