* Remove microtasks_scope.h and microtasks_scope.cc
* Use v8::MicrotasksScope when ignoring browser checkpoint
These call always skip the browser checkpoint, so they are equivalent to using v8::MicrotasksScope directly (modulo the optional wrapper behavior).
* Remove MicrotasksScope from node_bindings.cc
This code seems contradictory: it explicitly specifies "do not run microtasks" yet runs a microtask checkpoint in the browser process.
Looking at its history, it [was introduced][1] with the intention to not run microtasks, but a [subtle C++ language behavior][2] caused it to do the opposite later in the same roll. Since the original intention was to not run microtasks, and since that is also the simplest explanation, we can assume `ignore_browser_checkpoint` should be true and migrate this to `v8::MicrotasksScope` as it is equivalent (modulo the optional wrapper behavior).
[1]: a4ea80dd47 (diff-efe58cf03c97028f37f801db044d396a5f428686da6595d2c692f1c052bbd09c)
[2]: https://github.com/electron/electron/pull/43185
* Migrate gin_helper/promise.h and gin_helper/promise.cc to v8::MicrotasksScope
Restores the [original][1] behavior of running the microtask checkpoint at destruction, but preserves the behavior of running microtasks in the browser process. This had last changed in the migration to gin_helper::MicroTasks.
[1]: https://github.com/electron/electron/pull/16401
