Add unittest for canonical host override.

Currently, when adding an alternative service with a new host, that host will
become the canonical host (if appropriate), override previous canonical hosts
for the same domain.  This CL adds a test to document this behavior.

BUG=392575

Review URL: https://codereview.chromium.org/1006813015

Cr-Commit-Position: refs/heads/master@{#322356}
1 file changed