[heap] Fix TSan race in HeapTest.GrowAndShrinkNewSpace

Make sure there is no background GC when setting flags.

Bug: v8:12612, v8:13185
Change-Id: I0a2d4796abe265defa00d86f826003eb048e5bf1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3829482
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82474}

test/unittests/heap/heap-unittest.cc

@@ -10,6 +10,7 @@
 
 #include "include/v8-isolate.h"
 #include "include/v8-object.h"
+#include "src/common/assert-scope.h"
 #include "src/handles/handles-inl.h"
 #include "src/heap/gc-tracer.h"
 #include "src/heap/memory-chunk.h"
@@ -183,8 +184,17 @@ TEST_F(HeapTest, GrowAndShrinkNewSpace) {
   if (FLAG_single_generation) return;
   // Avoid shrinking new space in GC epilogue. This can happen if allocation
   // throughput samples have been taken while executing the benchmark.
-  FLAG_predictable = true;
-  FLAG_stress_concurrent_allocation = false;  // For SimulateFullSpace.
+  {
+    // Force an full GC and finish sweeping to make sure there is no active GC
+    // in the background while flags are updated.
+    CollectAllGarbage();
+    heap()->mark_compact_collector()->EnsureSweepingCompleted(
+        MarkCompactCollector::SweepingForcedFinalizationMode::kUnifiedHeap);
+    DisallowGarbageCollection no_gc_scope;
+    DCHECK(!heap()->incremental_marking()->IsRunning());
+    FLAG_predictable = true;
+    FLAG_stress_concurrent_allocation = false;  // For SimulateFullSpace.
+  }
   NewSpace* new_space = heap()->new_space();
 
   if (heap()->MaxSemiSpaceSize() == heap()->InitialSemiSpaceSize()) {