Add unittest for canonical host override.

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
Committed: https://crrev.com/6f210fa8372d05f03450e52d45deb194596b65f1
Cr-Commit-Position: refs/heads/master@{#322356}

[Bence, 2015-03-25 15:27:34]

bnc@chromium.org changed reviewers:
+ rch@chromium.org

[Bence, 2015-03-25 15:27:35]

Ryan, PTAL, thanks.

[Ryan Hamilton, 2015-03-26 00:24:57]

lgtm

[Bence, 2015-03-26 11:24:48]

The CQ bit was checked by bnc@chromium.org

[commit-bot: I haz the power, 2015-03-26 11:25:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1006813015/1

[commit-bot: I haz the power, 2015-03-26 11:28:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-26 11:29:07]

Patchset 1 (id:??) landed as
https://crrev.com/6f210fa8372d05f03450e52d45deb194596b65f1
Cr-Commit-Position: refs/heads/master@{#322356}