skau: Are you going to look at this? Want to flip this to assigned?

I don't have time to look at it right now.

What do you mean by capabilities fetch failing?

I experimented with this on a local build on Linux and tried a couple different things: 
(1) forcing GetCapabilitiesOnBlockingPoolThread to return a nullptr, which triggered the capabilities fetch failure event. I could still access the "Change" button to change the destination as well as the cancel button to close the dialog. I do think it would be good to display an error message here to let the user know they should select a different destination. The preview is stuck at "Loading" and none of the options appear so it is clear something is wrong, but not what exactly the problem is or how to fix it.
(2) Also tried forcing GetPrinterSemanticCapsAndDefaults to return false, which would result in a successful GetCapabilities call but an empty dictionary of printer capabilities. This also worked fine - actually printed successfully with this. It just won't necessarily populate all the preview dialog fields that are supported since it cannot see that those capabilities exist.

GetCapabilitiesOnBlockingPoolThread returning nullptr is the most obvious way to make it fail.  If I remember correctly, there's an error variable somewhere in the renderer that prevents printing or loading of any attributes after this stage.  The bug is in the javascript somewhere but I don't remember exactly where.

I think the unrecoverable error occurs when we have an invalid settings error from the native layer (which is usually due to something being incorrect about the printer setup), not when capabilities fetch fails entirely. This gets triggered by the PrintHostMsg_PrintPreviewInvalidPrinterSettings message sent from print web view helper.

The Print Preview UI state gets set to ERROR and then never escapes. When I tested this, it was still possible to change printers and settings and the preview updated, but as soon as I tried to print something it froze because we hit a check in the javascript.

Does that sound like what you were running into?

Yes.  That's the behavior I'm referring to.  Changing to a new printer should clear the error but I'm not sure why the current behavior was chosen.  There might be some state that needs to be reset to avoid bad behavior.  I haven't had the time to figure out why it behaves this way.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a7b31e1ff636ee016885a18953745783c9825616

commit a7b31e1ff636ee016885a18953745783c9825616
Author: rbpotter <rbpotter@chromium.org>
Date: Mon May 01 19:08:20 2017

Fix Print Preview failure state and add test

Prevent print preview error state from being unrecoverable by resetting
the state when the user selects a new printer. Add test to check that
the state resets and to check that the response to the INVALID_SETTINGS
event is correct.

BUG= 708834 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2849733002
Cr-Commit-Position: refs/heads/master@{#468379}

[modify] https://crrev.com/a7b31e1ff636ee016885a18953745783c9825616/chrome/browser/resources/print_preview/print_preview.js
[modify] https://crrev.com/a7b31e1ff636ee016885a18953745783c9825616/chrome/test/data/webui/print_preview.js

 Issue 708739  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d3b61aeab646c01a5e0a921a580e603f160678e3

commit d3b61aeab646c01a5e0a921a580e603f160678e3
Author: skau <skau@chromium.org>
Date: Wed May 03 01:07:56 2017

Reduce the log level for printer capability fetch failure.

If printer capability fetch fails, it is indicative of an error, but
it is recoverable.  So, it should be reduced to a warning.  This enables
testing of the failure case for printer capability retrieval.

BUG= 708834 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2850353002
Cr-Commit-Position: refs/heads/master@{#468850}

[modify] https://crrev.com/d3b61aeab646c01a5e0a921a580e603f160678e3/chrome/browser/resources/print_preview/data/destination_store.js
[modify] https://crrev.com/d3b61aeab646c01a5e0a921a580e603f160678e3/chrome/test/data/webui/print_preview_destination_search_test.js