Add [Exposed] to MutationObserver and DOMTokenList

[zqzhang]

As talked in https://github.com/heycam/webidl/issues/365, all interface X { ... } require [Exposed] to get rid of [PrimaryGlobal].

• [ ] https://github.com/whatwg/dom/commit/c9544325095195c3afdf6284c29b26e84ce572aa Add [Exposed=Window] to MutationObserver
• [ ] https://github.com/whatwg/dom/commit/da7b3ec5d9a4cbd857f0dc6c5ef164331ae156be Add [Exposed] to DOMTokenList

[zqzhang]

This will be addressed in next release.