Avoid unnecessary calls to leakRef() in Node

Using static_pointer_cast shouldn't be any scarier here than the
toMouseEvent/toTouchEvent calls, since we're checking the type tags on
the previous line in each cast.

R=abarth

Review URL: https://codereview.chromium.org/37083003

git-svn-id: svn://svn.chromium.org/blink/trunk@160373 bbb929c8-8fbe-4397-9dbb-9b2b20218538