Add ability to filter by form factor in VariationsService. 

The from factor API under /ui/base/form_factor.h is used to obtained the current device form factor. 


BUG=312920

Review URL: https://codereview.chromium.org/37243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234626 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234626