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With this change we add basic error reporting (to be improved with UMA
later) as well as more UI configuration options.

This essentially decouples the webview loading from the presentation of
the flow, so we don't flicker when we make async decisions. Example:
1: WebviewHost finishes load and reports header value.
2: WebviewHost shows empty webview contents.
3: Controller processes the value and decides to exit the flow.
4: The screen flickers since steps 2 and 3 happen too fast and are
   racing.

Introducing a presentation struct solves this issue and also helps to
implement another requirement, optional buttons for flow pages.

This leaves us with the Controller holding all the important state for
the flow, which allows us to write pretty detailed unit tests that cover
most issues. This will remove some of the burden from the browser_tests,
which are already too complex.

Bug: 958995
Change-Id: I5929688ee6a753f817b023937f5d56be597b9fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628035Reviewed-by: Oliver Chang <ochang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Lucas Tenório <ltenorio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665052}