[Files.app] Ensure the selected item be out of the back of the preview panel

BUG=303901
TEST=manually tested
R=hirono@chromium.org

Review URL: https://codereview.chromium.org/26205002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227264 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227264