VariationsService: Log reason why requests aren't allowed.

Extends existing Variations.ResourceRequestsAllowed histogram to
log the reason why requests weren't allowed.

BUG=321677

Review URL: https://codereview.chromium.org/78963002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236443 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236443