Oilpan: fix PromiseTracker's handling of its persistent promise wrappers.

The handling of the PromiseDataWrapper in r181662 wasn't complete, as it
failed to keep an Oilpan-reachable persistent to the GCed wrapper object,
risking premature finalization. For Oilpan also, this wrapper object should 
only keep its references weakly alive.

To make the tracing of these relationships work out, PromiseTracker is
no longer a part object, but heap allocated.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/561753002

git-svn-id: svn://svn.chromium.org/blink/trunk@181845 bbb929c8-8fbe-4397-9dbb-9b2b20218538