Refactor VariationsSeedStore class out of VariationsService.

This paves way for future changes to this class without
introducing more complexity in VariationsService.

BUG=none
TEST=New unit tests.

Review URL: https://codereview.chromium.org/133273024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245947 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245947