Use isSVG*Element() helpers more in SVG code (Part 2)

Use isSVG*Element() helpers more in SVG code. This makes the code
clearer / simpler.

R=pdr@chromium.org, pdr
BUG=346095

Review URL: https://codereview.chromium.org/192143002

git-svn-id: svn://svn.chromium.org/blink/trunk@168800 bbb929c8-8fbe-4397-9dbb-9b2b20218538