Oilpan: move PromiseTracker to the heap (fix build after r181646.)

Follow up r181646 and make PromiseTracker work better with Oilpan.

TBR=oilpan-reviews
BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/558673003

git-svn-id: svn://svn.chromium.org/blink/trunk@181662 bbb929c8-8fbe-4397-9dbb-9b2b20218538