Chrome-side cleanup now that WebHelperPlugin isn't a WebWidget.

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

Review URL: https://codereview.chromium.org/177213018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253607 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253607