This is working on current Dev so it's most likely a recent regression.

Indeed.
Description says Chrome Version: https://chromium.googlesource.com/chromium/src.git/+/831a1ec9ca49586b323d13d4e9b11c355deff761

According to https://chromiumdash.appspot.com/commit/831a1ec9ca49586b323d13d4e9b11c355deff761, it is in Canary for now (71.0.3563.0).

This bug requires manual review: We are only 11 days from stable.
Please contact the milestone owner if you have questions.
Owners: benmason@(Android), kariahda@(iOS), geohsu@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

As https://chromium-review.googlesource.com/c/chromium/src/+/1260129 landed in M70, we need this CL as well in M70 as it fixes an issue with the previous CL.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9dbabe98af77a8eab4fc53078182115c6e364c24

commit 9dbabe98af77a8eab4fc53078182115c6e364c24
Author: François Beaufort <beaufort.francois@gmail.com>
Date: Thu Oct 04 08:48:07 2018

[Picture-in-Picture] Do not always reset pip_player when entering PiP.

This CL makes sure pip_player does not get reset when entering PiP while
there was already another video playing in PiP mode.

Bug:  889850 
Change-Id: I468459584745bcd7eb21d282c52c4cbb8053fd74
Reviewed-on: https://chromium-review.googlesource.com/c/1249064
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#596547}
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/chrome/browser/chromeos/arc/pip/arc_picture_in_picture_window_controller_impl.cc
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/chrome/browser/chromeos/arc/pip/arc_picture_in_picture_window_controller_impl.h
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/chrome/browser/picture_in_picture/picture_in_picture_window_controller_browsertest.cc
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/chrome/browser/picture_in_picture/picture_in_picture_window_manager.cc
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/chrome/browser/picture_in_picture/picture_in_picture_window_manager.h
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/chrome/browser/ui/views/overlay/overlay_window_views.cc
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/chrome/browser/ui/views/overlay/overlay_window_views.h
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/content/browser/picture_in_picture/picture_in_picture_window_controller_impl.cc
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/content/browser/picture_in_picture/picture_in_picture_window_controller_impl.h
[modify] https://crrev.com/9dbabe98af77a8eab4fc53078182115c6e364c24/content/public/browser/picture_in_picture_window_controller.h

How safe is this merge overall? I see that another CL landed in #6, has this been tested in canary?

Approving merge. This is behind a flag. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a3efec324bebbfab0c532caa88cbf7f76750e339

commit a3efec324bebbfab0c532caa88cbf7f76750e339
Author: François Beaufort <beaufort.francois@gmail.com>
Date: Fri Oct 05 08:24:46 2018

[Picture-in-Picture] Do not always reset pip_player when entering PiP.

This CL makes sure pip_player does not get reset when entering PiP while
there was already another video playing in PiP mode.

Bug:  889850 
Change-Id: I468459584745bcd7eb21d282c52c4cbb8053fd74
Reviewed-on: https://chromium-review.googlesource.com/c/1249064
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Original-Commit-Position: refs/heads/master@{#596547}(cherry picked from commit 9dbabe98af77a8eab4fc53078182115c6e364c24)
Reviewed-on: https://chromium-review.googlesource.com/c/1264201
Reviewed-by: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/branch-heads/3538@{#872}
Cr-Branched-From: 79f7c91a2b2a2932cd447fa6f865cb6662fa8fa6-refs/heads/master@{#587811}
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/chrome/browser/chromeos/arc/pip/arc_picture_in_picture_window_controller_impl.cc
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/chrome/browser/chromeos/arc/pip/arc_picture_in_picture_window_controller_impl.h
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/chrome/browser/picture_in_picture/picture_in_picture_window_controller_browsertest.cc
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/chrome/browser/picture_in_picture/picture_in_picture_window_manager.cc
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/chrome/browser/picture_in_picture/picture_in_picture_window_manager.h
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/chrome/browser/ui/views/overlay/overlay_window_views.cc
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/chrome/browser/ui/views/overlay/overlay_window_views.h
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/content/browser/picture_in_picture/picture_in_picture_window_controller_impl.cc
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/content/browser/picture_in_picture/picture_in_picture_window_controller_impl.h
[modify] https://crrev.com/a3efec324bebbfab0c532caa88cbf7f76750e339/content/public/browser/picture_in_picture_window_controller.h

The following revision refers to this bug: 
https://chromium.googlesource.com/chromium/src.git/+/a3efec324bebbfab0c532caa88cbf7f76750e339

Commit: a3efec324bebbfab0c532caa88cbf7f76750e339
Author: beaufort.francois@gmail.com
Commiter: beaufort.francois@gmail.com
Date: 2018-10-05 08:24:46 +0000 UTC

[Picture-in-Picture] Do not always reset pip_player when entering PiP.

This CL makes sure pip_player does not get reset when entering PiP while
there was already another video playing in PiP mode.

Bug:  889850 
Change-Id: I468459584745bcd7eb21d282c52c4cbb8053fd74
Reviewed-on: https://chromium-review.googlesource.com/c/1249064
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Original-Commit-Position: refs/heads/master@{#596547}(cherry picked from commit 9dbabe98af77a8eab4fc53078182115c6e364c24)
Reviewed-on: https://chromium-review.googlesource.com/c/1264201
Reviewed-by: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/branch-heads/3538@{#872}
Cr-Branched-From: 79f7c91a2b2a2932cd447fa6f865cb6662fa8fa6-refs/heads/master@{#587811}

Verified in 71.0.3575.1 (Official Build) canary (64-bit)