Started investigating this, because it seems related to https://bugs.chromium.org/p/chromium/issues/detail?id=652666.

Started with a simple approach of not loading <settings-prefs> when in Guest mode, see [1]. This fixes non-CrOS runtime exceptions.

With [1], in CrOS guest mode, there is still a runtime exception which is caused by calling chrome.settingsPrivate.setDefaultZoomLevel(), which is always erroneously called at startup (this has already been discovered at at  crbug.com/652483 ).

@stevenjb, michaelpg: Is not loading <settings-prefs> the correct approach for guest mode? Un-assigning myself for now.

[1] https://codereview.chromium.org/2406043002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7577ff2509a7a5d91ec319f12a42dd1acdb1a41f

commit 7577ff2509a7a5d91ec319f12a42dd1acdb1a41f
Author: dpapad <dpapad@chromium.org>
Date: Sat Oct 15 00:41:22 2016

MD Settings: Stop calling chrome.settingsPrivate.setDefaultZoomLevel on startup.

BUG= 652483 , 654588 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2403353002
Cr-Commit-Position: refs/heads/master@{#425519}

[modify] https://crrev.com/7577ff2509a7a5d91ec319f12a42dd1acdb1a41f/chrome/browser/resources/settings/appearance_page/appearance_page.html
[modify] https://crrev.com/7577ff2509a7a5d91ec319f12a42dd1acdb1a41f/chrome/browser/resources/settings/appearance_page/appearance_page.js

Ping @stevenjb @michaelpg, see question at #2.

<settings-prefs> is required for Chrome OS.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/09a6d19d7bf0552dfc879828014fefd68fccdef2

commit 09a6d19d7bf0552dfc879828014fefd68fccdef2
Author: hcarmona <hcarmona@chromium.org>
Date: Wed Nov 16 17:43:52 2016

Fix Guest Mode errors in MD Settings by using the incognito context.

BUG= 654588 

Review-Url: https://codereview.chromium.org/2499093002
Cr-Commit-Position: refs/heads/master@{#432544}

[modify] https://crrev.com/09a6d19d7bf0552dfc879828014fefd68fccdef2/chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.cc
[modify] https://crrev.com/09a6d19d7bf0552dfc879828014fefd68fccdef2/chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.h