Oilpan: fix g++ (4.8) building after r180659.

Pin down the resolution of + to be over int by converting the enum value.

R=haraken,zerny
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/488923004

git-svn-id: svn://svn.chromium.org/blink/trunk@180774 bbb929c8-8fbe-4397-9dbb-9b2b20218538