In chrome://discards, add 'Can Freeze?'/'Can Discard?' columns

The 'Can Discard?' column indicates proactive discards, not urgent discards.

Each cell in those columns shows a checkmark or crossmark, with a
'[View Reason]' link. Hovering the link shows a tooltip with the reasons why
it can't be frozen/discarded.

The 'View Reason' button is disabled if the tab _can_ be frozen/discarded, since
there are no failure reasons to show.

Also, remove the 'Media' column, since it's redundant now.

Screenshot (apologies for external contributors):
https://screenshot.googleplex.com/n9oDCgM3ffj

Bug: 864149
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ied07835a5eae7a80989226a3662dc12d3e9ee9b2
Reviewed-on: https://chromium-review.googlesource.com/1140368
Commit-Queue: S├ębastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: S├ębastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576216}
8 files changed