Remove NOTATION, ENTITY, and XPATH_NAMESPACE as valid node types

None of these can be created in Blink, so having them appear in
switch(nodeType) statements is just confusing. Luckily, Blink
already has a solution for that: DeprecatedNodeType.

Also removes these from the Blink WebNode API; there are no references
to them in Chromium.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/132103003

git-svn-id: svn://svn.chromium.org/blink/trunk@164806 bbb929c8-8fbe-4397-9dbb-9b2b20218538