Make a new member function EventHandler::handleMouseFocus with focus-related code in dispatchMouseEvent.

handleMouseFocus should be called only for mousedown case. We don't call it in
dispatchMouseEvent, and calls it after dispatchMouseEvent(
EventTypeNames::mousedown, ...).

BUG=360454
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/229423003

git-svn-id: svn://svn.chromium.org/blink/trunk@171197 bbb929c8-8fbe-4397-9dbb-9b2b20218538