Oilpan: fix build after r180653.

Need an explicit RawPtr<> conversion to resolve toV8() overloading.

TBR=oilpan-reviews
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/493783002

git-svn-id: svn://svn.chromium.org/blink/trunk@180671 bbb929c8-8fbe-4397-9dbb-9b2b20218538