Avoid branching in createAttributeEventListener

Avoid branching in createAttributeEventListener, also it should not be needed to test for SVG specific stuff in this spot.

Review URL: https://codereview.chromium.org/321023002

git-svn-id: svn://svn.chromium.org/blink/trunk@175902 bbb929c8-8fbe-4397-9dbb-9b2b20218538