Oilpan: make r180096's FIXME comment accurate.

The underlying cause described in r180096 wasn't correct; update the
FIXME comment to make it more accurate.

R=haraken
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/461373002

git-svn-id: svn://svn.chromium.org/blink/trunk@180147 bbb929c8-8fbe-4397-9dbb-9b2b20218538