Blink used to use v8::MicrotasksScope::GetCurrentDepth() to get the
number of nested MicrotasksScope for the default microtask queue.
However, there was no corresponding one for non-default queues.

Change-Id: I1c2472ba19b1a11cb968f02119d91d92867c6e02
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1567705Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60862}