Linux: call xdg-open on downloaded files to open them.

BUG=12299
TEST=1) right click/save as on some image. Clicking on the download item's filename area should launch it in some image viewer (assuming xdg-open works for you---as it happens, ubuntu broke xdg-open for desktops that are not gnome, kde, or xfce). 2) Download a large file. Click on the download item before it is finished. The text should change to "opening in..." and the checkbox in the dropdown menu should show as checked. 3) completed downloads should have the "open when finished" menu item replaced by the "open" menu item. Selecting that should also open the download.
Review URL: http://codereview.chromium.org/112064

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@17106 0039d316-1c4b-4281-b951-d872f2087c98