Chrome Version: 70 (r586980)
OS: Linux, Windows

What steps will reproduce the problem?
(Debug build only.)
(1) Open a hosted app window.
(2) (Linux) Change theme from GTK to non-GTK or vice versa.
(2) (Windows 7) Change OS theme from Glass/Aero to Basic or vice versa.
(2) (Windows 10) Change OS theme from high-contrast to normal or vice versa.

What is the expected result?
Nothing breaks.

What happens instead?
[3068:3088:0829/011601.168:FATAL:browser_view.cc(1010)] Check failed: !toolbar_button_provider_.

NOTE: This situation isn't covered by any tests, but I'm currently trying to add a test that hits it. So this is blocking me from landing that test. See https://crrev.com/c/1195226.

The DCHECK in question is in SetToolbarButtonProvider:
https://cs.chromium.org/chromium/src/chrome/browser/ui/views/frame/browser_view.cc?l=1035

The documentation for SetToolbarButtonProvider:

  // Sets the button provider for this BrowserView. Must be called before
  // InitViews() which sets the ToolbarView as the default button provider.
  void SetToolbarButtonProvider(ToolbarButtonProvider* provider);

This comment backs up the DCHECK, saying that you shouldn't be setting the toolbar button provider *after* the class is initialized.

However, alancutter@ and I can't see any reason to have this requirement. Setting a new toolbar button provider is something that we do, today, in production (when switching graphical modes), and it seems to properly clean up the old one and set the new one.

Therefore, I think we should just remove the DCHECK and the above comment.