Update HTMLCollection.namedItem() to use the id/name cache. HTMLCollection
maintains an id/name cache internally. It was used by namedItems() method but
strangely not by the namedItem() one.

This change is good for performance as HTMLCollection.namedItem() will now use
the cache when available and thus avoid traversing the DOM tree unnecessarily.

There should be not web-exposed behavior change. This functionality is covered
by a decent amount of layout tests.

The CL also gets rid of HTMLCollection::checkForNameMatch() method as it is no
longer used. HTMLCollection::updateNameCache() already has the same checks
inlined.

R=arv, adamk

Review URL: https://codereview.chromium.org/154183002

git-svn-id: svn://svn.chromium.org/blink/trunk@166498 bbb929c8-8fbe-4397-9dbb-9b2b20218538