[Extensions] Make list of activities scroll within itself

This CL extends the height of the activity log to be 100% page height
instead of being as tall as its contents and allows overflow to be
applied. In this case, the activity lists in both the history and
stream tab scroll within themselves, which should make them more usable
with the rest of the page.

This is also a preparatory CL for iron-list as having the activity log
stream's height be set by its parent means that the iron-list can resize
with the page but still be bound in height, which is required for it to
render only items that are visible and for appearance.

Screenshots: https://imgur.com/a/RfhG2XU

Bug: 832354
Change-Id: I6de6d049a18c8c0f20b96722d5e2b0af74af0190
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526565
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642647}
3 files changed