Oilpan: add missing HTMLFormControlsCollection tracing.

R=haraken@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/298053002

git-svn-id: svn://svn.chromium.org/blink/trunk@174765 bbb929c8-8fbe-4397-9dbb-9b2b20218538