Refactored synchronizeAttribute to remove duplicated code.
Fixed a small inconsistency in the two methods. In particular, equalPossiblyIgnoringCase is now called in the synchronizeAttribute variant that uses a QualifiedName, whereas it was not before. This fixes a potential issue in that we were ignoring namespaces.

BUG=322239

Review URL: https://codereview.chromium.org/100723003

git-svn-id: svn://svn.chromium.org/blink/trunk@163729 bbb929c8-8fbe-4397-9dbb-9b2b20218538