Mark localName() / prefix() / namespaceURI() virtual methods as FINAL

Mark localName() / prefix() / namespaceURI() virtual methods as FINAL in
Element as these are never overriden in Element subclasses. They have
different implementations in Attr and Element only, and the Attr class is
already marked as FINAL.

This increases the chances these methods are inlined by the compiler and
thus help with performance as these are hot in some benchmarks. Some callers
such as HTMLTagNodeList::nodeMatchesInlined() are hot and already know the
type is an Element at compile time.

R=adamk, abarth

Review URL: https://codereview.chromium.org/121793005

git-svn-id: svn://svn.chromium.org/blink/trunk@164380 bbb929c8-8fbe-4397-9dbb-9b2b20218538