[heap] Ensure that non-regular chunks are freed before scavenge

This prevents accumulation of non-regular chunks if unmapper tasks
are not making progress.

Bug: chromium:934453
Change-Id: I552bc4f566f4be8877d9e806cca2aa9c284a7f4f
Reviewed-on: https://chromium-review.googlesource.com/c/1483055Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59842}

src/heap/heap.cc

@@ -639,6 +639,7 @@ void Heap::GarbageCollectionPrologue() {
     ephemeron_retainer_.clear();
     retaining_root_.clear();
   }
+  memory_allocator()->unmapper()->PrepareForGC();
 }
 
 size_t Heap::SizeOfObjects() {

src/heap/mark-compact.cc

@@ -796,8 +796,6 @@ void MarkCompactCollector::Prepare() {
     heap()->incremental_marking()->Stop();
   }
 
-  heap()->memory_allocator()->unmapper()->PrepareForMarkCompact();
-
   if (!was_marked_incrementally_) {
     TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_EMBEDDER_PROLOGUE);
     heap_->local_embedder_heap_tracer()->TracePrologue();

src/heap/spaces.cc

@@ -307,8 +307,7 @@ void MemoryAllocator::Unmapper::CancelAndWaitForPendingTasks() {
   }
 }
 
-void MemoryAllocator::Unmapper::PrepareForMarkCompact() {
-  CancelAndWaitForPendingTasks();
+void MemoryAllocator::Unmapper::PrepareForGC() {
   // Free non-regular chunks because they cannot be re-used.
   PerformFreeMemoryOnQueuedNonRegularChunks();
 }

src/heap/spaces.h

@@ -1250,7 +1250,7 @@ class V8_EXPORT_PRIVATE MemoryAllocator {
 
     V8_EXPORT_PRIVATE void FreeQueuedChunks();
     void CancelAndWaitForPendingTasks();
-    void PrepareForMarkCompact();
+    void PrepareForGC();
     void EnsureUnmappingCompleted();
     V8_EXPORT_PRIVATE void TearDown();
     size_t NumberOfCommittedChunks();