[unified-consent] Revamp advanced sync setup during opt-in

- Add 'Cancel' and 'Confirm' buttons to sync account control
  during the sync setup.
- Make cancel the default action when the user leaves the
  sync setup without confirming.
- Remove snackbar from sync controls page.

Video:
https://drive.google.com/file/d/1tjn6Ey6ddbDff9xwOu9hP2-bJv-3b77U/view?usp=sharing

Bug: 919745
Change-Id: Ibf83e00cd4a7d7e3aab5eaf55e6bd2cdc17309cd
Reviewed-on: https://chromium-review.googlesource.com/c/1422000
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#626355}(cherry picked from commit 1503157f8de7ecae2cc25d366e393e972e020057)
Reviewed-on: https://chromium-review.googlesource.com/c/1449678
Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#115}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
13 files changed