[filemanager] Open Downloads by default (instead of removable device).

BUG=chromium-os:29914
TEST=manual
Review URL: https://chromiumcodereview.appspot.com/10198010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@133674 0039d316-1c4b-4281-b951-d872f2087c98