Use stricter typing for HTMLCollection named getter

Use stricter typing for HTMLCollection named getter. According to specification,
it should return an Element, not a Node. This CL updates the IDL and the native
side to explicitly use Element type.

There is no Web-Exposed behavior change. The stricter typing just makes clearer
what kind of Nodes we are dealing with.

R=haraken, adamk

Review URL: https://codereview.chromium.org/148013008

git-svn-id: svn://svn.chromium.org/blink/trunk@166041 bbb929c8-8fbe-4397-9dbb-9b2b20218538