Add WebNode::getElementsByHTMLTagName.

The function will replace existing WebNode::getElementsByTagName.

BUG=394234

Review URL: https://codereview.chromium.org/393983006

git-svn-id: svn://svn.chromium.org/blink/trunk@178259 bbb929c8-8fbe-4397-9dbb-9b2b20218538