events/views: Add a null-check on Window::delegate() before accessing it.

BUG=336015

Review URL: https://codereview.chromium.org/140413004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245841 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245841