Use isSVG*Element() helpers more in SVG code (Part 3)

Use isSVG*Element() helpers more in SVG rendering code. Also use the
Traversal<SVG*Element> API where suitable. This makes the code clearer
and simpler.

R=pdr
BUG=346095

Review URL: https://codereview.chromium.org/191003007

git-svn-id: svn://svn.chromium.org/blink/trunk@168808 bbb929c8-8fbe-4397-9dbb-9b2b20218538