When running microtasks, the auto policy currently only invokes
MicrotasksCompletedCallbacks when the microtask queue is non-empty,
while all other policies unconditionally invokes the callbacks. Make the
auto mode also unconditionally invoke the callbacks.

Bug: v8:10213
Change-Id: I2f608459960b84e6f506646712ac935130646b9e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2057813Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66320}