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A follow-up CL will create the blocking page and controller client
in ChromeWebClient to show the UI.

Bug: 1058898
Change-Id: I1709a6bba4accd22089ea24ef5183a6101c27bdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261364Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784543}