[SVG] Remove existing event listeners when eventBase conditions are cleared

This patch fixes a regression from [1] where we are not removing event listeners when eventBase conditions are cleared.

[1] http://src.chromium.org/viewvc/blink?view=rev&revision=164697

R=pdr@chromium.org, schenney@chromium.org, dschulze@chromium.org, fmalita@chromium.org, rob.buis@samsung.com
BUG=338124

Review URL: https://codereview.chromium.org/145583005

git-svn-id: svn://svn.chromium.org/blink/trunk@165880 bbb929c8-8fbe-4397-9dbb-9b2b20218538