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This implements the Quiet Billing interstitial and adds the approved
strings from http://crbug.com/887186 and adds the interstitial to the
debug WebUI page (chrome://interstitials).

WebView support will come in a follow-up CL.

Bug: 887186
Test: browser_tests --gtest_filter=InterstitialUITest.BillingInterstitialQuiet
Test: Manual - observe chrome://interstitials page
Change-Id: I4ff9811d925bdde81bf1349ac714c2533ad563bb
Reviewed-on: https://chromium-review.googlesource.com/1256021Reviewed-by: Carlos IL <carlosil@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595642}