r448615 in  Issue 689085  decoupled the task manager, but didn't add a new flag for it.

Marking this as Phase4, but it doesn't necessarily need to block things there. Maybe this is even just `Code Health` since we get to delete a .xib, but it's not really getting in the way or requiring much maintenance as it is.

I ended up doing this because it was bothering me: https://codereview.chromium.org/2724923004/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/63762e98af73be680ff4d8237cee0e2240faf397

commit 63762e98af73be680ff4d8237cee0e2240faf397
Author: ellyjones <ellyjones@chromium.org>
Date: Wed Mar 15 14:04:29 2017

cocoa: add ViewsTaskManager feature

This feature uses the Views task manager instead of the Cocoa one. It looks strange
unless you also use --secondary-ui-md to get the Harmony controls.

BUG= 670579 

Review-Url: https://codereview.chromium.org/2724923004
Cr-Commit-Position: refs/heads/master@{#457073}

[modify] https://crrev.com/63762e98af73be680ff4d8237cee0e2240faf397/chrome/app/generated_resources.grd
[modify] https://crrev.com/63762e98af73be680ff4d8237cee0e2240faf397/chrome/browser/about_flags.cc
[modify] https://crrev.com/63762e98af73be680ff4d8237cee0e2240faf397/chrome/browser/ui/cocoa/task_manager_mac.mm
[modify] https://crrev.com/63762e98af73be680ff4d8237cee0e2240faf397/chrome/common/chrome_features.cc
[modify] https://crrev.com/63762e98af73be680ff4d8237cee0e2240faf397/chrome/common/chrome_features.h
[modify] https://crrev.com/63762e98af73be680ff4d8237cee0e2240faf397/tools/metrics/histograms/histograms.xml