Cleanup now that WebHelperPlugin no longer needs to be a WebWidget.

BUG=346543
R=abarth@chromium.org, ddorwin@chromium.org

Review URL: https://codereview.chromium.org/181953002

git-svn-id: svn://svn.chromium.org/blink/trunk@167936 bbb929c8-8fbe-4397-9dbb-9b2b20218538