Document that WebNode's subclasses must not add any data members.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@156939 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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