Don't call Chrome::focusedNodeChanged() for detached document

This patch changes Document::setFocusedElement() not to call Chrome:focusedNodeChanged() when Document::page() returns null.

This situation can be happened when event handler removes document during focus related events, blur, focus, and so on, dispatched in Document::setFocusedElement().

BUG=249164
TEST=LayoutTests/fast/events/blur-remove-parent-crash.html
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/23463009

git-svn-id: svn://svn.chromium.org/blink/trunk@156908 bbb929c8-8fbe-4397-9dbb-9b2b20218538