Reland "[heap] Start sweeper tasks after evacuation" (second try).

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index 3a794a8c6b4417ad4c7faa5b14d3d0510634d3d3..26b29afaeb9816397ad37fb26b671960e6510b72 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -395,11 +395,12 @@ void MemoryAllocator::Unmapper::ReconsiderDelayedChunks() {
 
 bool MemoryAllocator::CanFreeMemoryChunk(MemoryChunk* chunk) {
   MarkCompactCollector* mc = isolate_->heap()->mark_compact_collector();
-  // We cannot free memory chunks in new space while the sweeper is running
-  // since a sweeper thread might be stuck right before trying to lock the
-  // corresponding page.
-  return !chunk->InNewSpace() || (mc == nullptr) || !FLAG_concurrent_sweeping ||
-         mc->sweeper().IsSweepingCompleted();
+  // We cannot free a memory chunk in new space while the sweeper is running
+  // because the memory chunk can be in the queue of a sweeper task.
+  // Chunks in old generation are unmapped if they are empty.
+  DCHECK(chunk->InNewSpace() || chunk->SweepingDone());
+  return !chunk->InNewSpace() || mc == nullptr || !FLAG_concurrent_sweeping ||
+         mc->sweeper().IsSweepingCompleted(NEW_SPACE);
 }
 
 bool MemoryAllocator::CommitMemory(Address base, size_t size,