file_manager: Move logic for rejecting google documents to the right place

And add a unit test.

BUG=269929
TEST=none
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/26972002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228138 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228138