Oilpan: improve comment on why types need to be moved into WebCore.

Clarify why the explicit move of some type names into WebCore is
needed & label it a FIXME.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/164713002

git-svn-id: svn://svn.chromium.org/blink/trunk@167119 bbb929c8-8fbe-4397-9dbb-9b2b20218538