[heap] Use forced flags for LowMemoryNotification

The notification is only used for testing and benchmarking purposes.
Announcing low memory is usually done using MemoryPressure API.

Bug: chromium:843903
Change-Id: I998018f7f5f3a0d06283aa6010228a9c86f12c39
Reviewed-on: https://chromium-review.googlesource.com/c/1269037Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56454}

src/heap/heap.cc

@@ -1124,9 +1124,12 @@ void Heap::CollectAllAvailableGarbage(GarbageCollectionReason gc_reason) {
   isolate_->compilation_cache()->Clear();
   const int kMaxNumberOfAttempts = 7;
   const int kMinNumberOfAttempts = 2;
+  const v8::GCCallbackFlags callback_flags =
+      gc_reason == GarbageCollectionReason::kLowMemoryNotification
+          ? v8::kGCCallbackFlagForced
+          : v8::kGCCallbackFlagCollectAllAvailableGarbage;
   for (int attempt = 0; attempt < kMaxNumberOfAttempts; attempt++) {
-    if (!CollectGarbage(OLD_SPACE, gc_reason,
-                        v8::kGCCallbackFlagCollectAllAvailableGarbage) &&
+    if (!CollectGarbage(OLD_SPACE, gc_reason, callback_flags) &&
         attempt + 1 >= kMinNumberOfAttempts) {
       break;
     }