Retain the controller in -[DownloadItemButton rightMouseDown:] before opening the menu that targets it.

BUG=129826
TEST=See comment #18 in bug.


Review URL: https://chromiumcodereview.appspot.com/10543033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@140870 0039d316-1c4b-4281-b951-d872f2087c98