Eliminate "FIXME: Oilpan ..."s

This CL eliminates "FIXME: Oilpan ..."s that are easy to eliminate.
I'll address more non-trivial ones in follow-up CLs.

BUG=340522

Review URL: https://codereview.chromium.org/353613002

git-svn-id: svn://svn.chromium.org/blink/trunk@176824 bbb929c8-8fbe-4397-9dbb-9b2b20218538