[MutationObservers] Microbenchmarks for appendChild, removeChild, and innerHTML

[MutationObservers] Microbenchmarks for appendChild, removeChild, and innerHTML
https://bugs.webkit.org/show_bug.cgi?id=71939

Reviewed by Ojan Vafai.

These benchmarks time both the DOM mutations themselves and the time taken
to call the MutationCallback. This measurement is achieved by
executing the code entirely within the MutationCallback itself.

* Mutation/append-child-deep.html: Added. Covers appendChild in a 200-node-deep tree with subtree observation.
* Mutation/append-child.html: Added. Covers appendChild in a flat tree.
* Mutation/inner-html.html: Added. Covers innerHTML adding and removing many nodes at a time.
* Mutation/remove-child-deep.html: Added. Covers removeChild in a 200-node-deep tree with subtree observation.
* Mutation/remove-child.html: Added. Covers removeChild in a flat tree.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@99879 268f45cc-cd09-0410-ab3c-d52691b4dbfc