Oilpan: fix build after r351868.

We do not have compiler support for traceable Oilpan heap references,
hence undo the Member<> to ref conversion that r351868 made.

TBR=oilpan-reviews
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/1370143007

Cr-Commit-Position: refs/heads/master@{#351887}
4 files changed