Have getElementsByClassName() / getElementsByTagName*() return an HTMLCollection

Have getElementsByClassName() / getElementsByTagName*() return an HTMLCollection
instead of a NodeList, as per the specification:
http://dom.spec.whatwg.org/#document
http://dom.spec.whatwg.org/#element

This behavior is consistent with Firefox 26 and IE 11.

R=arv, haraken, tkent
BUG=235008

Review URL: https://codereview.chromium.org/143453010

git-svn-id: svn://svn.chromium.org/blink/trunk@166088 bbb929c8-8fbe-4397-9dbb-9b2b20218538