[MutationObservers] Remove platform-dependent code in Document.cpp resulting from Mutation Event histogram collection
https://bugs.webkit.org/show_bug.cgi?id=73026

Patch by Rafael Weinstein <rafaelw@chromium.org> on 2011-12-16
Reviewed by Ryosuke Niwa.

This patch adds platform/HistogramSupport which has an empty implementation for all ports
except Chromium.

No tests need. This patch is just a refactor.

* GNUmakefile.list.am:
* Target.pri:
* WebCore.gypi:
* WebCore.xcodeproj/project.pbxproj:
* dom/Document.cpp:
(WebCore::histogramMutationEventUsage):
(WebCore::Document::~Document):
* platform/HistogramSupport.h: Added.
(WebCore::HistogramSupport::histogramEnumeration):
* platform/chromium/HistogramSupportChromium.cpp: Added.
(WebCore::HistogramSupport::histogramEnumeration):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@103131 268f45cc-cd09-0410-ab3c-d52691b4dbfc