Use HTMLElementFactory to create equivalent elements in WebVTTElement.

This change forces the calls through the factory so that we create
appropriate sub-classes based upon the passed tag name, rather than
just creating a concrete HTMLElement class with an inappropriate tag
name.

BUG=243881
R=inferno@chromium.org, vcarbune@chromium.org

Review URL: https://codereview.chromium.org/16019013

Patch from Tom Sepez <tsepez@chromium.org>.

git-svn-id: svn://svn.chromium.org/blink/trunk@151287 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed