Make downloads pinnable to the toolbar (part 1)

This cl adds a temporary flag for this series of downloads toolbar
button changes which moves downloads into the
PinnedToolbarActionsContainer, hides the old downloads toolbar button
when the flag is enabled, creates an actionitem for downloads, creates a
new controller for handling downloads toolbar ui related updates
(previously handled by the toolbar button view itself), and lets the
new pinnable toolbar button open the downloads chrome page on click.

Follow up work includes, making this work for webapps, hooking up
DownloadBubbleNavigationHandler and DownloadBubbleRowListViewInfoObserver
for showing the recent downloads bubble for the new toolbar button, and
adding various button related animations and badging.

Bug: 323962334
Change-Id: I777285866acde0d608c240f8934c1cc7474cf026
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6138712
Reviewed-by: David Pennington <dpenning@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1403883}