Remove old API from WebLeakDetector.

There is no users of it on either Blink or Chromium. This fixes the
FIXME in WebLeakDetector.h.

BUG=None
TEST=None
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/428473002

git-svn-id: svn://svn.chromium.org/blink/trunk@179002 bbb929c8-8fbe-4397-9dbb-9b2b20218538