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For lookalike interstitials, call CancelAndDisplayError from
ShouldAllowResponse rather than at the request stage, so that the
provisional load has already started. This will allow PrepareErrorPage
to trigger.

This CL also puts a placeholder method in ChromeWebClient for preparing
the lookalike interstitial HTML when it sees a lookalike error code.

Bug: 1058898
Change-Id: Iff830bb48cdee60cc1e2b8b5f74ebe858c2d5993
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261614Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783085}