Document that WebNode's subclasses must not add any data members.

This requirement allows code using classes in this heirarchy to safely
static_cast to the appropriate subclass based on the actual type of the wrapped
node.

BUG=278295
TEST=none
R=darin@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23672019

git-svn-id: svn://svn.chromium.org/blink/trunk@156939 bbb929c8-8fbe-4397-9dbb-9b2b20218538