Use new Traversal<*Element> API in svg code for clarity

Use new Traversal<*Element> API in svg code for clarity. It results in shorter /
simpler code. It also makes it clearer what type of Element we are actually
interested in.

R=
BUG=346733

Review URL: https://codereview.chromium.org/205983004

git-svn-id: svn://svn.chromium.org/blink/trunk@169621 bbb929c8-8fbe-4397-9dbb-9b2b20218538