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With this change, we are showing the Supervision Onboarding screens right
after the ARC ToS screen. We are also skipping any screens that show after
a successful Supervision Onboarding.

This change also removes the ExitFlow() method for the WebviewHost
interface. Since this data was just looping back to the WebUI implementation,
we can just add a new C++ interface that bypasses the JS bridge.

TBR=ochang@chromium.org

Bug: 958995
Change-Id: I340c3899e8ca3d0bdb493ca487e682600467212a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1653388
Commit-Queue: Lucas Tenório <ltenorio@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668538}