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This CL proceeds with the happy path of the signed-in profile creation
flow:
 1) After a successful sign-in, the current WebUI for sync confirmation
    gets displayed;
 2) After choosing whether to sync, the flow is finished by opening a
    browser window for the new profile.

Error handling and restyling the confirmation page is left for follow-up
CLs.

Bug: 1126913
Change-Id: I17b43fe4ec0c0e0f2f9725b1df8e8e59d2c35f3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421592
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811140}