Avoid updating state of DownloadItemView after it's been deleted.

BUG=349081

Review URL: https://codereview.chromium.org/193333002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256716 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256716