Don't assert when clicking on a <button> and then switching to a different app.

BUG=364304
TEST=code is less trigger-happy
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/240433003

git-svn-id: svn://svn.chromium.org/blink/trunk@171831 bbb929c8-8fbe-4397-9dbb-9b2b20218538