Use getAttribute instead of fastGetAttribute for SVG animated types

While there is currently no attribute of SVG animated type marked to
[Reflect], adding a check to avoid future problems would be as much
code as adding proper support for it.

IsSVGAnimatedType() sunbroutine is also simplified so that we don't
need to hardcode the SVG Animated types anymore.

Review URL: https://chromiumcodereview.appspot.com/19540003

git-svn-id: svn://svn.chromium.org/blink/trunk@154389 bbb929c8-8fbe-4397-9dbb-9b2b20218538