bindings: Retires ScriptWrappable::init, etc.

Retires ScriptWrappable::init and renames m_wrapperOrTypeInfo to m_wrapper
because we no longer need WrapperTypeInfo in m_wrapperOrTypeInfo.
Removes related code, too.

BUG=235436

Review URL: https://codereview.chromium.org/540283003

git-svn-id: svn://svn.chromium.org/blink/trunk@181507 bbb929c8-8fbe-4397-9dbb-9b2b20218538