Simplify SVGTextContentElement::elementFromRenderer()

Simplify SVGTextContentElement::elementFromRenderer() by leveraging
SVGElement::isTextContent() methods instead of explicitly checking
for tag names.

Review URL: https://chromiumcodereview.appspot.com/18358002

git-svn-id: svn://svn.chromium.org/blink/trunk@153348 bbb929c8-8fbe-4397-9dbb-9b2b20218538
CSP: "Value values" should be "Valid values" in 'reflected-xss' error message.

Review URL: https://chromiumcodereview.appspot.com/18323002

git-svn-id: svn://svn.chromium.org/blink/trunk@153349 bbb929c8-8fbe-4397-9dbb-9b2b20218538