On Chrome for iOS with a signed-in user, this non-Google page can force
native sign-in UI / an incognito tab to open by navigating (top-level or
in a subframe) to an accounts.google.com endpoint that returns the
X-Chrome-Manage-Accounts header.
AccountConsistencyHandler::ShouldAllowResponse acts on the header without
checking the request initiator or response_info.for_main_frame.
Top-level navigation variants:
Subframe variant (also fires — for_main_frame is ignored):