This adds a new API function that can be customized by the embedder
by providing a delegate that defines contexts to be measured and
reports the results to JS.

A memory measurement request is carried out as follows:

1) MeasureMemory(delegate) invocation enqueues a new request in
   MemoryMeasurement::received_ and schedules a delayed GC task.

2) At the start of the next GC (that is triggered either by the
   GC schedule or by the delayed task) each request in received_
   moves to processing_. Per-context marking worklists are created
   for each native context that was selected by the delegates
   (using the ShouldMeasure predicate).

3) At the end of the GC the sizes of the native contexts are
   recorded for each request in processing_. The requests move
   to the done_ list and result reporting task is scheduled.

4) When the result reporting task runs it invokes the
   MeasurementComplete function of each delegate in done_.


Bug: chromium:973627

Change-Id: I0254cae693c5b8fab7c85a9eca0a3a128210b6c4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1981493
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65856}