Since the unified system tray has been enabled by default and the logic of old system tray has been deleted (e.g, https://chromium-review.googlesource.com/c/chromium/src/+/1307354). I think the code that added for dragging the system tray menu/ stylus/ IME/ notifications are not needed now. I will go to remove the related logic, like the clipping window, tray drag controller etc.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c332890948b6d9f0b10a61c3688b9a933ecc0618

commit c332890948b6d9f0b10a61c3688b9a933ecc0618
Author: Min Chen <minch@google.com>
Date: Fri Nov 16 18:47:18 2018

Clean up gesture drag for system tray, IME etc.

Unified system tray has been launched and web notification has been moved
to the top of the system tray. The old system tray logic has been removed.
This cl removes the logic that swiping up on the status area tray to open
the corresponding bubble and swiping down on the tray bubble to close it.

cl that landed the feature:
https://codereview.chromium.org/2961313003/
cl that disabled the feature:
https://chromium-review.googlesource.com/c/chromium/src/+/674527/

Bug:  767679 
Change-Id: Id81091360ee47c9763d2f716f246146b940ac1e2
Reviewed-on: https://chromium-review.googlesource.com/c/1334932
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608868}
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/BUILD.gn
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/metrics/user_metrics_action.h
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/metrics/user_metrics_recorder.cc
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/palette/palette_tray.cc
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/tray/tray_background_view.cc
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/tray/tray_background_view.h
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/tray/tray_bubble_view.cc
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/tray/tray_bubble_view.h
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/tray/tray_event_filter_unittest.cc
[delete] https://crrev.com/b24393f3c2dd478d3fb7d80d79b881f256a587d8/ash/system/tray_drag_controller.cc
[delete] https://crrev.com/b24393f3c2dd478d3fb7d80d79b881f256a587d8/ash/system/tray_drag_controller.h
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/unified/unified_slider_bubble_controller.cc
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/unified/unified_system_tray.cc
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/ash/system/unified/unified_system_tray_bubble.cc
[modify] https://crrev.com/c332890948b6d9f0b10a61c3688b9a933ecc0618/tools/metrics/actions/actions.xml