Fix the crash in ShelfView::UpdateDragIconProxy.

BUG=347222
R=skuhne@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/185003004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254317 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254317