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When reloading a page whose url leads to an error page, a different
pathway is taken in NavigationControllerImpl. We must be careful to
check for unreachable urls in
NavigationControllerImpl::RendererDidNavigateToExistingPage() in this
case, and mark them as error pages.

BUG=458036

Review URL: https://codereview.chromium.org/917043004

Cr-Commit-Position: refs/heads/master@{#317046}