Add toSVGFEFuncR|G|B|AElement() fuctions, and use them.

These inline functions will check if type casting is correct with assertions.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23463030

git-svn-id: svn://svn.chromium.org/blink/trunk@157863 bbb929c8-8fbe-4397-9dbb-9b2b20218538