[MICRODATA] Build failure in html/HTMLPropertiesCollection.h

[MICRODATA] Build failure in html/HTMLPropertiesCollection.h
https://bugs.webkit.org/show_bug.cgi?id=90379

Patch by Christophe Dumez <christophe.dumez@intel.com> on 2012-07-02
Reviewed by Ryosuke Niwa.

Switch HTMLCollection::append() visibility from private
to protected so that HTMLPropertiesCollection subclass
can call it. This fixes build when MICRODATA flag is
turned on.

No new tests, build fix.

* html/HTMLCollection.h:
(HTMLCollectionCacheBase):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121689 268f45cc-cd09-0410-ab3c-d52691b4dbfc