Use isSVG*Element() helpers more in SVG code (Part 1)

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

R=pdr
BUG=346095

Review URL: https://codereview.chromium.org/192133002

git-svn-id: svn://svn.chromium.org/blink/trunk@168796 bbb929c8-8fbe-4397-9dbb-9b2b20218538