Use FeatureObserver to see how often web sites use multipart main documents
https://bugs.webkit.org/show_bug.cgi?id=111015

Reviewed by Nate Chapin.

Multipart main documents add sigificant complexity to the loader.  It
would be interesting to know how often this complexity is used.

* loader/MainResourceLoader.cpp:
(WebCore::MainResourceLoader::responseReceived):
* page/FeatureObserver.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144250 268f45cc-cd09-0410-ab3c-d52691b4dbfc