A pointer to MicrotaskQueue is stored in a NativeContext field as a Smi,
that is discouraged. This CL replaces it to use the dedicated field.

Bug: v8:8124
Change-Id: I5a770624b3a9c922051e86243da2ae216aaacf3a
Reviewed-on: https://chromium-review.googlesource.com/c/1351855
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57870}