Make SVGTRefElement use the modern Shadow DOM API

We can simplify SVGTRefElement by using modern Shadow DOM abstractions like
userAgentShadowRoot() and utility functions like replaceChildrenWithText.

R=pdr@chromium.org,dglazkov@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19870004

git-svn-id: svn://svn.chromium.org/blink/trunk@154809 bbb929c8-8fbe-4397-9dbb-9b2b20218538