Remove inefficiency from HTMLCollection::namedItems()

Remove inefficiency from HTMLCollection::namedItems(). The code was
unnecessarily checking that the pointers were non-null for every iteration of
the for loops. This CL updates the code so that the null check is done only
once.

R=esprehn@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/337963002

git-svn-id: svn://svn.chromium.org/blink/trunk@176247 bbb929c8-8fbe-4397-9dbb-9b2b20218538