Add ProfileInfoCache::ProfileIsChild/IsLegacySupervisedAtIndex.

Add ProfileInfoCache::ProfileIsChild/IsLegacySupervisedAtIndex.
Make sure there is always a non-LegacySupervised profile (it's okay if there is only a child profile!)

TBRing since this has already been reviewed at https://chromereviews.googleplex.com/108027013/
TBR=bauerb@chromium.org
TBR=noms@chromium.org
BUG=372381
Committed: https://crrev.com/4887f082a62b414ef3a611eb58690240aed1c408
Cr-Commit-Position: refs/heads/master@{#307229}

[Marc Treib, 2014-12-08 11:39:08]

The CQ bit was checked by treib@chromium.org

[commit-bot: I haz the power, 2014-12-08 11:39:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/777993003/1

[commit-bot: I haz the power, 2014-12-08 12:37:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-08 12:38:16]

Patchset 1 (id:??) landed as
https://crrev.com/4887f082a62b414ef3a611eb58690240aed1c408
Cr-Commit-Position: refs/heads/master@{#307229}

[Marc Treib, 2014-12-08 12:48:46]

treib@chromium.org changed reviewers:
+ bauerb@chromium.org, noms@chromium.org

[Marc Treib, 2014-12-08 12:48:46]

Adding TBRed reviewers as actual reviewers, just fyi.