The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cd10773b9d9bdbfea20f33f6418c4b3825ad9041

commit cd10773b9d9bdbfea20f33f6418c4b3825ad9041
Author: Jennifer Apacible <apacible@chromium.org>
Date: Wed May 16 01:55:21 2018

[Picture in Picture] Enable window drag.

Currently, the Picture-in-Picture window cannot be dragged by just
interacting with a mouse (click-and-hold-to-drag). On Linux, it is
possible to drag by also holding down the alt button.

This change allows dragging the window if the user gesture is not on
the controls (i.e. play/pause, close).

Manually tested on Mac OSX and Linux.

Bug:  840540 
Change-Id: Ie3008d41ede2788d508c5cace47b7b062448f979
Reviewed-on: https://chromium-review.googlesource.com/1058376
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: apacible <apacible@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558922}
[modify] https://crrev.com/cd10773b9d9bdbfea20f33f6418c4b3825ad9041/chrome/browser/ui/views/overlay/overlay_window_views.cc