This breakage seems to be scoped to --login-manager usage.
Also, pinning apps from the app list in --mash with --login-manager is broken.
(the shelf items wind up having no icon nor title and do nothing on clicks...)

Actually, it seems like extensions won't even launch from --mash with --login-manager.
Clicking Files in the app list (w/ --disable-features=EnableFullscreenAppList) does nothing.
(it works just fine with --mash but not using the login manager)

Discovered after I found that ExtensionRegistry::GetExtensionById fails in --mash with --login-manager.
I wonder if this is a profile/prefs issue? Did it used to work?
The ability to bisect would be nice, but I can't as per  issue 764487 ...

Do we know when ChromeLauncherController::AttachProfile is called after user sign-in?

For classic ash, this seems happening in ChromeLauncherController ctor and from Shell::InitShelf after session state becomes ACTIVE [1]. That is, after user sign-in.

However for mash, ChromeLauncherController is created earlier in ChromeBrowserMainExtraPartsAsh::PostProfileInit [2]. With --login-manager, PostProfileInit is called after sign-in profile is loaded. Hence ChromeLauncherController is attached to the sign-in profile. The problem is that after sign-in, I don't see ChromeLauncherController::AttachProfile gets called to change it to the user profile. 

[1]: https://cs.chromium.org/chromium/src/ash/shell.cc?rcl=d86e751bd876008156d79a977d388b74777216fa&l=1231

[2]: https://cs.chromium.org/chromium/src/chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc?rcl=6550b0a9b59558fd2a2190cf7950e3ab897905d8&l=135

Attaching to the signin profile would explain all the symptoms (pinning in classic doesn't affect mash, pinning in mash works if you stay in mash because it's reading the stored signin profile, etc.).

That's a good lead, I'll take a look tomorrow. Thanks for the insights!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/077c49e54a20c8cd198f85c7f36e6bb728edcb24

commit 077c49e54a20c8cd198f85c7f36e6bb728edcb24
Author: Mike Wasserman <msw@chromium.org>
Date: Thu Sep 14 17:16:15 2017

mash: Create ChromeLauncherController on session activation, like cash.

Init mash's CLC in ChromeBrowserMainPartsChromeos when the session to becomes active.
(cash: Shell::OnSessionStateChanged calls ChromeShellDelegate::InitializeShelf/ShelfInit)

TODO: Fix mash crash on V1 app launch as a tab ( crbug.com/764960 )
TODO: Move cash ChromeLauncherController init to this same pattern.

Bug:  764394 
Change-Id: I985c32656a0b2bfdaa29fe31e9d5ca8df3fa1326
Reviewed-on: https://chromium-review.googlesource.com/666288
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501976}
[modify] https://crrev.com/077c49e54a20c8cd198f85c7f36e6bb728edcb24/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
[modify] https://crrev.com/077c49e54a20c8cd198f85c7f36e6bb728edcb24/chrome/browser/chromeos/chrome_browser_main_chromeos.h
[modify] https://crrev.com/077c49e54a20c8cd198f85c7f36e6bb728edcb24/chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc
[modify] https://crrev.com/077c49e54a20c8cd198f85c7f36e6bb728edcb24/chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.h