[Files.app] Use a custom WebStore URL even when the mime is not given.

A custom WebStore URL should be used even when the mime is not given. But the non-custom URL is used when the mime is empty due to bug. This patch fixes it.

BUG=303865
TEST=mannual

Review URL: https://codereview.chromium.org/25483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226887 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226887