[heap] Ensure progress in unmapping memory chunks.

If sweeping is not making progress and there are many young generation
GCs happening, then this can lead to accumulation of memory chunks in
the unmapper queue.

Bug: chromium:771966
Change-Id: Ief73ada0d17198a80b668850c6d2e7ea413113e7
Reviewed-on: https://chromium-review.googlesource.com/702479Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48312}

src/heap/heap.cc

@@ -1935,6 +1935,12 @@ void Heap::Scavenge() {
   IncrementalMarking::PauseBlackAllocationScope pause_black_allocation(
       incremental_marking());
 
+  if (mark_compact_collector()->sweeper().sweeping_in_progress() &&
+      memory_allocator_->unmapper()->NumberOfDelayedChunks() >
+          kMaxSemiSpaceSizeInKB / Page::kPageSize) {
+    mark_compact_collector()->EnsureSweepingCompleted();
+  }
+
   mark_compact_collector()->sweeper().EnsureNewSpaceCompleted();
 
   SetGCState(SCAVENGE);

src/heap/spaces.h

@@ -1196,6 +1196,11 @@ class V8_EXPORT_PRIVATE MemoryAllocator {
 
     bool has_delayed_chunks() { return delayed_regular_chunks_.size() > 0; }
 
+    int NumberOfDelayedChunks() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      return static_cast<int>(delayed_regular_chunks_.size());
+    }
+
    private:
     static const int kReservedQueueingSlots = 64;
     static const int kMaxUnmapperTasks = 24;