Once CppHeap moves away from EmbedderHeapTracer, it can no longer be
returned by Isolate::GetEmbedderHeapTracer. Blink currently uses
GetEmbedderHeapTracer to trigger GCs. This CL introduces a variant of
Isolate::RequestGarbageCollectionForTesting to replace that.

Bug: v8:12407
Change-Id: I099989f55291a0daab7d2051072dedddef58dd40
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3293413
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78015}