Use isSVG*Element() helpers more in SVG code (Part 4)

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

R=pdr
BUG=346095

Review URL: https://codereview.chromium.org/190793011

git-svn-id: svn://svn.chromium.org/blink/trunk@168807 bbb929c8-8fbe-4397-9dbb-9b2b20218538