Protect autofocus element in AutofocusTask::performTask.

Document::m_autofocusElement can have the last reference to the element.

BUG=335921
TEST=automated
R=keishi@chromium.org

Review URL: https://codereview.chromium.org/132053005

git-svn-id: svn://svn.chromium.org/blink/trunk@165378 bbb929c8-8fbe-4397-9dbb-9b2b20218538