What would happen if the timeout occurs? Will the app never be installed, or will it try again next startup?

Also what happens if there's no timeout... do we just have a WebContents loading in the background forever? (It shouldn't block anything else in the OS, right?)

The app will never be installed, and it will forever block any other apps added to the pending queue for installation. This is independent of user-triggered install so it won't block that, but it's still kind of bad that one stalled load will fail all default installs / policy installs. I'd say this should be a launch blocker.

I'll take this one.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/385952ee3baf6430b0b54b7d6f1734c823f812fb

commit 385952ee3baf6430b0b54b7d6f1734c823f812fb
Author: Dominick Ng <dominickn@chromium.org>
Date: Wed Aug 22 10:56:18 2018

Add a timeout for loading the provided URL in PendingBookmarkAppManager.

This CL ensures that loads which never finish do not stall the pending
task queue. Tests are added to ensure the timer functions as expected.

BUG= 876172 

Change-Id: I65477d202a846de2687f9ccf48dd243f26a7d80a
Reviewed-on: https://chromium-review.googlesource.com/1184732
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584984}
[modify] https://crrev.com/385952ee3baf6430b0b54b7d6f1734c823f812fb/chrome/browser/web_applications/extensions/pending_bookmark_app_manager.cc
[modify] https://crrev.com/385952ee3baf6430b0b54b7d6f1734c823f812fb/chrome/browser/web_applications/extensions/pending_bookmark_app_manager.h
[modify] https://crrev.com/385952ee3baf6430b0b54b7d6f1734c823f812fb/chrome/browser/web_applications/extensions/pending_bookmark_app_manager_unittest.cc