src: start heap object tracking after platform is initialized

Garbage collection is triggered by the heap object tracker. The garbage
collector may spawn garbage collection tasks on the foreground thread.
This is only allowed after registering the isolate in the platform
which happens in IsolateData. This CL moves the starting of the heap
object tracker after calling IsolateData to ensure that.

PR-URL: https://github.com/nodejs/node/pull/17249


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>