The flag is used by V8 to propagate information on whether this GC was
somehow forced.

This allows the embedder using EmbedderHeapTracer to easily figure out
whether a GC was forced based on a local flag without depending on GC
prologue/epilogue callbacks.

Bug: chromium:1074061
Change-Id: Ic04c93ae58da854e54c6da1ca4aac03b5ab49897
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2182473Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67567}