Oilpan: Prpare to move WorkerEventQueue to Oilpan heap.

BUG=340522,375671

Review URL: https://codereview.chromium.org/303133005

git-svn-id: svn://svn.chromium.org/blink/trunk@175148 bbb929c8-8fbe-4397-9dbb-9b2b20218538