Mark error pages as such on Ctrl-Shift-T reload.

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.


Review URL:

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