Make not to crash for mouse down event on pseudo element and removing associate node

This patch adds null check for event target node in shouldRefetchEventTarget(), which called by EventHandler::handleMousePressEvent(), rather than assuming target node isn't null.

In EventHandler::handleMousePressEvent(), we can have null target node when mouse down is on pseudo element, e.g. :before/:after CSS selector, and removing associated node during 'mousedown' handler.

BUG=304175
TEST=LayoutTests/fast/events/mouse-down-on-pseudo-element-remove-crash.html
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/26603005

git-svn-id: svn://svn.chromium.org/blink/trunk@159208 bbb929c8-8fbe-4397-9dbb-9b2b20218538