Add MutationObserverInit

To remove generic Dictionary from MutationObserver.
Added Vector<String> -> HashSet<AtomicString> conversion
because attributeFilter member in MutationObserverInit
is sequence<DOMString>.

Tests under fast/dom/MutationObserver cover this change.

Spec: http://www.w3.org/TR/dom/#interface-mutationobserver

BUG=403150

Review URL: https://codereview.chromium.org/560003002

git-svn-id: svn://svn.chromium.org/blink/trunk@181881 bbb929c8-8fbe-4397-9dbb-9b2b20218538