Update WebNode::getElementsByTagName so that it returns only HTML elements.

A temporary change to fix a bug. All of callsites of this function in
Chromium expect that it returns only HTML elements.
The function name and the behavior doesn't match. We should fix this
inconsistency soon.

BUG=387315
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/351763003

git-svn-id: svn://svn.chromium.org/blink/trunk@176819 bbb929c8-8fbe-4397-9dbb-9b2b20218538