This prevents inexperienced users from creating website compatibility
problems by blocking cookies.

As this is the first supervised user content setting that is ALLOW
rather than BLOCK, rename content_settings::BinaryValueMap to
GlobalValueMap and add support for arbitrary content settings.

Also some cleanup, most importantly:
* Remove the AutoLock from the map, as it doesn't need to hold on to the
  lock beyond the scope of a method call.
* Return a null RuleIterator when there is no content setting, which
  improves performance because it avoids memory churn.

BUG=678278

Review-Url: https://codereview.chromium.org/2628883008
Cr-Commit-Position: refs/heads/master@{#444071}