getElementsByClassName should include non styled Elements

Allow Elements who are not styled but have class set to be retrieved using getElementsByClassName.

Matches FF behavior.

https://bugs.webkit.org/show_bug.cgi?id=94718

Review URL: https://codereview.chromium.org/458663003

git-svn-id: svn://svn.chromium.org/blink/trunk@180414 bbb929c8-8fbe-4397-9dbb-9b2b20218538