For default browser:
We can sort of copy what settings is doing[1]. Because the default-check is async, we can do the following:

1) on the welcome-page load, fire off the check and store a promise.
2) block the modules' initialization until the promise is resolved.

This way we can guarantee to know if we need the set-as-default module before deciding what modules to initialize, and this most likely won't block for too long anyway (currently loads almost instantly for settings)


[1]https://cs.chromium.org/chromium/src/chrome/browser/ui/webui/settings/settings_default_browser_handler.cc?type=cs&g=0&l=72

Also for default browser, check this:
https://cs.chromium.org/chromium/src/chrome/browser/ui/startup/default_browser_prompt.cc?type=cs&g=0&l=143

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/55b998d61e32c2ceb1c0552c757d1471c9b54319

commit 55b998d61e32c2ceb1c0552c757d1471c9b54319
Author: Scott Chen <scottchen@chromium.org>
Date: Thu Oct 25 22:18:58 2018

Nux Onboarding: don't show set-default step if already default or not allowed.

This CL makes the welcome_app check if the browser can be set as default, and
hold off on initializing modules until we find out.

Bug:  874122 ,  874255 
Change-Id: I42e8427a200c717c2eacea04ad9932d15e65caba
Reviewed-on: https://chromium-review.googlesource.com/c/1297283
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602896}
[modify] https://crrev.com/55b998d61e32c2ceb1c0552c757d1471c9b54319/chrome/browser/resources/welcome/onboarding_welcome/BUILD.gn
[modify] https://crrev.com/55b998d61e32c2ceb1c0552c757d1471c9b54319/chrome/browser/resources/welcome/onboarding_welcome/set_as_default/nux_set_as_default.js
[modify] https://crrev.com/55b998d61e32c2ceb1c0552c757d1471c9b54319/chrome/browser/resources/welcome/onboarding_welcome/set_as_default/nux_set_as_default_proxy.js
[modify] https://crrev.com/55b998d61e32c2ceb1c0552c757d1471c9b54319/chrome/browser/resources/welcome/onboarding_welcome/shared/nux_types.js
[modify] https://crrev.com/55b998d61e32c2ceb1c0552c757d1471c9b54319/chrome/browser/resources/welcome/onboarding_welcome/welcome_app.html
[modify] https://crrev.com/55b998d61e32c2ceb1c0552c757d1471c9b54319/chrome/browser/resources/welcome/onboarding_welcome/welcome_app.js