This CL merges MicrotaskContext management into EnteredContext, so that
MicrotaskContext can nest.

Here is a brief explanation:
https://docs.google.com/document/d/1MY_xlsYS7E6_qbwwY66-FH3JkAYeTHBlF5qVBrBpWyY/edit

Benchmark result:
No significant performance difference is observed for this CL.
See "patched" row for the result of this CL. Maintaining |is_microtask_context_|
for the deprecated Isolate::GetEnteredContext() seems to cost 1~2% of the
score, but that will be resolved eventually.
https://github.com/v8/promise-performance-tests
https://docs.google.com/spreadsheets/d/1-SpO4nQNxcXQZAfHN5CmEyAyCBd33wZ_CdF4U78e44I/edit#gid=1701841321

Bug: v8:8124
Change-Id: Ic709bccba9c32d37578e15a7571014ce50129459
Reviewed-on: https://chromium-review.googlesource.com/c/1322290Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58174}