[MutationObservers] Add histogram collection for usage of DOM Mutation Events

[MutationObservers] Add histogram collection for usage of DOM Mutation Events
https://bugs.webkit.org/show_bug.cgi?id=72316

Patch by Rafael Weinstein <rafaelw@chromium.org> on 2011-11-14
Reviewed by Ryosuke Niwa.

This patch adds six calls in ~Document() which simply pipe-out to the embedder
the (already-collected) bits of whether varous DOM Mutation Events were registered
on the document.

No tests needed. No functional changes.

* dom/Document.cpp:
(WebCore::histogramMutationEventUsage):
(WebCore::Document::~Document):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@100222 268f45cc-cd09-0410-ab3c-d52691b4dbfc