Refactoring: Simplify EventHandler::dispatchMouseEvent.

The main purpose of this CL is replacing "Node* node" with
"Element* element" because only elements can be focusable.

BUG=258763

Review URL: https://chromiumcodereview.appspot.com/20621002

git-svn-id: svn://svn.chromium.org/blink/trunk@155163 bbb929c8-8fbe-4397-9dbb-9b2b20218538