Check isSVGElement() instead of element namespace in RL::isTransparent()

This is a merge of http://trac.webkit.org/changeset/157360 by
Andreas Kling <akling@apple.com>

RenderLayer::isTransparent is visible on profiles and the namespace
check for SVG is unnecessary. This patch replaces it with a check
that the element is an SVG element.

BUG=309423

Review URL: https://codereview.chromium.org/30883002

git-svn-id: svn://svn.chromium.org/blink/trunk@160032 bbb929c8-8fbe-4397-9dbb-9b2b20218538