heap: Introduce EmbedderHeapTracer::kForced

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}

include/v8.h

@@ -7842,6 +7842,7 @@ class V8_EXPORT EmbedderHeapTracer {
   enum TraceFlags : uint64_t {
     kNoFlags = 0,
     kReduceMemory = 1 << 0,
+    kForced = 1 << 2,
   };
 
   // Indicator for the stack state of the embedder.

src/heap/heap.cc

@@ -5464,8 +5464,11 @@ EmbedderHeapTracer* Heap::GetEmbedderHeapTracer() const {
 }
 
 EmbedderHeapTracer::TraceFlags Heap::flags_for_embedder_tracer() const {
-  if (ShouldReduceMemory())
+  if (is_current_gc_forced()) {
+    return EmbedderHeapTracer::TraceFlags::kForced;
+  } else if (ShouldReduceMemory()) {
     return EmbedderHeapTracer::TraceFlags::kReduceMemory;
+  }
   return EmbedderHeapTracer::TraceFlags::kNoFlags;
 }