IDL: Prefer to extend IdlTypeBase instead of IdlType

In v8_types.py and elsewhere, we extend the classes defined in idl_types.py
by adding properties or methods. Usually, these properties or methods are
supposed to be available on all type objects. Since all type objects now
have a common base class, IdlTypeBase, prefer to set new properties and
methods on it instead of on IdlType.

This doesn't change CG, and is done to simplify the process of adding new
IdlTypeBase sub-classes in the future.

Review URL: https://codereview.chromium.org/469243002

git-svn-id: svn://svn.chromium.org/blink/trunk@180276 bbb929c8-8fbe-4397-9dbb-9b2b20218538