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|IsWiFiConnection| isn't accurate on some platforms. Whenever SSID is available,
use it and ignore |IsWiFiConnection|. This CL also adds a delegate to
|CaptivePortalBlockingPage| for better testing.

BUG=451272

Review URL: https://codereview.chromium.org/894153003

Cr-Commit-Position: refs/heads/master@{#314391}