Flush watermarks after compacting map space on mark-sweep collection.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 4f2d07b0c8176118fca2722d135e6da097a50b34..6165255fd47a27c89b8db68325f38e404edbb94d 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -2121,6 +2121,12 @@ class MapSpace : public FixedSpace {
     accounting_stats_.DeallocateBytes(accounting_stats_.Size());
     accounting_stats_.AllocateBytes(new_size);
 
+    // Flush allocation watermarks.
+    for (Page* p = first_page_; p != top_page; p = p->next_page()) {
+      p->SetAllocationWatermark(p->AllocationTop());
+    }
+    top_page->SetAllocationWatermark(new_top);
+
 #ifdef DEBUG
     if (FLAG_enable_slow_asserts) {
       intptr_t actual_size = 0;