use webstore extent to verify gallery downloads

      also relax test when command-line is used to allow empty referrers

BUG=56795,54408
TEST=install an app from a /webstore URL

Review URL: http://codereview.chromium.org/3616009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@61960 0039d316-1c4b-4281-b951-d872f2087c98