Introduce toSVGLinearGradientElement(), and use it

As a step to clean-up static_cast<SVGXXX>, toSVGLinearGradientElement() is added to clean-up
static_cast<SVGLinearGradientElement*>.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23205014

git-svn-id: svn://svn.chromium.org/blink/trunk@156442 bbb929c8-8fbe-4397-9dbb-9b2b20218538