Introduce toSVGViewElement(), and refactor setupInitialView() using it

As a step to clean-up static_cast<SVGXXX>, toSVGTextElement() is added to clean-up
static_cast<SVGTextElement*>. Besides this cl does refactoring duplicated condition checking
in setupInitialView().

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23636006

git-svn-id: svn://svn.chromium.org/blink/trunk@157206 bbb929c8-8fbe-4397-9dbb-9b2b20218538