Hide isXXX() type checking methods in subclasses

Hide isXXX() type checking methods in subclasses in order to catch unnecessary
calls at compile time. For e.g., it makes no sense to call
Element::isElementNode() as it is obviously true.

This CL also takes care of removing several such unnecessary calls that were
caught at compile time.

R=eseidel@chromium.org, pdr@chromium.org

Review URL: https://codereview.chromium.org/422463002

git-svn-id: svn://svn.chromium.org/blink/trunk@178921 bbb929c8-8fbe-4397-9dbb-9b2b20218538