The call starts: https://cs.chromium.org/chromium/src/chrome/browser/ui/webui/welcome_handler.cc?q=welcome+handler+cc&dr=CSs&l=64

and ends here:
https://cs.chromium.org/chromium/src/chrome/browser/signin/process_dice_header_delegate_impl.cc?dr=CSs&g=0&l=85

We should be able to pass the desired destination from the welcome_handler via the constructor call to the delegate.

the code changes for a prototype can be seen here: 
https://chromium-review.googlesource.com/c/chromium/src/+/1159347/7..9

more notes regarding implementation: we might have to store user's email provider selection in window.localStorage, to be used when the sign-in flow redirects back to chrome://welcome/go-to-email. The WelcomeHandler is reconstructed so there's not an easily accessible place to store this in the C++ process.

see: https://chromium-review.googlesource.com/c/chromium/src/+/1159347/9..10

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cc3f32c94248290c029ae944c5fd597d657caad2

commit cc3f32c94248290c029ae944c5fd597d657caad2
Author: Scott Chen <scottchen@chromium.org>
Date: Thu Sep 27 01:44:41 2018

NUX Onboarding: add a way to specify redirect url after sign-in.

In the NUX Onboarding welcome flow, it's necessary to direct the users back
to the chrome://welcome page after they finish signing in. Since the page
navigates to a remote location when the user is signing in, the specified URL
has to be stored in the sign-in helper in the backend.

Bug:  874147 
Change-Id: I39126f1072d172fcec023de160889ac7f4ecb7b4
Reviewed-on: https://chromium-review.googlesource.com/1239353
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594559}
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/resources/welcome/onboarding_welcome/landing_view.js
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/resources/welcome/onboarding_welcome/welcome_browser_proxy.js
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/signin/chrome_signin_helper.cc
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/signin/dice_tab_helper.cc
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/signin/dice_tab_helper.h
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/signin/dice_tab_helper_unittest.cc
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/signin/process_dice_header_delegate_impl.cc
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/signin/process_dice_header_delegate_impl.h
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/ui/signin_view_controller.cc
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/ui/signin_view_controller.h
[modify] https://crrev.com/cc3f32c94248290c029ae944c5fd597d657caad2/chrome/browser/ui/webui/welcome/welcome_handler.cc

The capability has been added and ready to be used by the NUX modules.