The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4

commit cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4
Author: Wez <wez@chromium.org>
Date: Wed Jun 13 16:29:12 2018

Fix content_shell quit correctly when the final Shell window closes.

This was broken by
https://chromium-review.googlesource.com/c/chromium/src/+/1090219
which failed to add the necessary link between the default //content
main loop runner and the Shell's quit closure.

Bug:  851416 
Change-Id: Ib69c9da55fbcc6143031cce0bf3d5e16087e6e21
Reviewed-on: https://chromium-review.googlesource.com/1096475
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566861}
[modify] https://crrev.com/cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4/content/browser/browser_main_loop.cc
[modify] https://crrev.com/cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4/content/public/browser/browser_main_parts.h
[modify] https://crrev.com/cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4/content/shell/browser/shell.cc
[modify] https://crrev.com/cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4/content/shell/browser/shell.h
[modify] https://crrev.com/cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4/content/shell/browser/shell_browser_main_parts.cc
[modify] https://crrev.com/cbf4a04cb05c29ada98f036ab4ae6dcdd4c9c1f4/content/shell/browser/shell_browser_main_parts.h

This should now be fixed - please re-open if you encounter related issues, though.

Wohoo! Thanks a lot! It no longer hangs here.