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The method is duplicated by sublasses. Push it to the base class instead.
An upcoming CL will use this for CaptivePortalBlockingPage as well.

Change-Id: Id24993df5fa0f7a0371e01ee84bf4788a196aca6
Reviewed-on: https://chromium-review.googlesource.com/902805Reviewed-by: Carlos IL <carlosil@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534519}