Oilpan: Fix RawPtr to Member in MediaKeySession::PendingAction

BUG=402766, 403121
NOTRY=true

Review URL: https://codereview.chromium.org/463413002

git-svn-id: svn://svn.chromium.org/blink/trunk@180140 bbb929c8-8fbe-4397-9dbb-9b2b20218538