The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bb9a744c80ca3187020a07168f7807f4f836ef2d

commit bb9a744c80ca3187020a07168f7807f4f836ef2d
Author: Noel Gordon <noel@chromium.org>
Date: Tue Sep 18 04:34:21 2018

AudioPlayer initialization: improve DOMContentLoaded handling

DOMContentLoaded can fire before the addEventListener is installed, so
setup the listener per convention: test "document.readyState" and then
install the event listener if needed.

Per the HTML DOM spec, DOMContentLoaded fires on document, not window,
so install the event listener on document.

Test: browser_tests --gtest_filter="AudioPlayerBrowserTest*"
Bug:  884963 
Change-Id: I0c8168dd4869d16ccfdf7dd01513d3921fe45c9b
Reviewed-on: https://chromium-review.googlesource.com/1229713
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591935}
[modify] https://crrev.com/bb9a744c80ca3187020a07168f7807f4f836ef2d/ui/file_manager/audio_player/js/audio_player.js

The VideoPlayer has the same issue as the AudioPlayer.  Changing title.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/99d369035a5137fc3039aa8cf599545824f49892

commit 99d369035a5137fc3039aa8cf599545824f49892
Author: Noel Gordon <noel@chromium.org>
Date: Thu Sep 20 05:48:05 2018

VideoPlayer initialization: await DOMContentLoaded

Make the initialization promise await DOMContentLoaded and then apply
i18n transforms to the video player DOM [1].

Minor: correct commentary, add commentary. Add assertions re document
readyState when the i18n template strings are applied.

[1] Applying i18n strings to an incomplete (readyState === 'loading')
DOM document makes no sense at all.

Test: browser_tests --gtest_filter="VideoPlayerBrowserTest*"
Bug:  884963 
Change-Id: Ie5a25763c54c56e949edde40c7e06b8eaad230c4
Reviewed-on: https://chromium-review.googlesource.com/1233394
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592690}
[modify] https://crrev.com/99d369035a5137fc3039aa8cf599545824f49892/ui/file_manager/video_player/js/video_player.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bf30bb4f3ce8301881a14a394383da5e5dc3c60d

commit bf30bb4f3ce8301881a14a394383da5e5dc3c60d
Author: Luciano Pacheco <lucmult@chromium.org>
Date: Thu Sep 20 09:12:05 2018

Include utils for Polymer2 and remove /deep/

Bug:  875525 ,  884963 
Change-Id: I3b3d290e2f7cc9781b7af3d3caea4508330e21ff
Reviewed-on: https://chromium-review.googlesource.com/1234118
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592729}
[modify] https://crrev.com/bf30bb4f3ce8301881a14a394383da5e5dc3c60d/ui/file_manager/audio_player/audio_player.html
[modify] https://crrev.com/bf30bb4f3ce8301881a14a394383da5e5dc3c60d/ui/file_manager/integration_tests/audio_player/open_audio_files.js

FTR,  issue 887792  filed for remaining parts of FilesApp and DOMContentLoaded.