In the spec, WeakRefs that are dereferenced are kept alive until there's
no JS on the stack, and then the host is expected to call
ClearKeptObjects to clear those strong references [1]. HTML calls
ClearKeptObjects at the end of a PerformMicrotaskCheckpoint [2].

In V8, leaving this up to the embedder is error prone in the same way
the deprecated FinalizationGroup callback APIs were error prone: it
depends on the embedder doing the right thing. This CL moves the call to
ClearKeptObjects to be after running of microtasks within V8.

However, the Isolate::ClearKeptObjects API should not be removed or
deprecated in case an embedder uses an entirely custom MicrotaskQueue
implementation and invokes MicrotaskQueue::PerformCheckpoint manually.

[1] https://tc39.es/proposal-weakrefs/#sec-clear-kept-objects
[2] https://github.com/whatwg/html/pull/4571

Bug: v8:8179
Change-Id: Ie243804157b56241ca69ed8fad300e839a0c9f75
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2055967
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66327}