improve allocation accounting for incremental mark

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index e9b23e58befee2a4c62f44382a9f629339cfdb83..493f8e6d6a96580b87575b69f15a265fa1995b87 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -2366,7 +2366,8 @@ class NewSpace : public Space {
         to_space_(heap, kToSpace),
         from_space_(heap, kFromSpace),
         reservation_(),
-        inline_allocation_limit_step_(0) {}
+        inline_allocation_limit_step_(0),
+        top_on_previous_step_(0) {}
 
   // Sets up the new space using the given chunk.
   bool SetUp(int reserved_semispace_size_, int max_semi_space_size);
@@ -2548,7 +2549,7 @@ class NewSpace : public Space {
   void LowerInlineAllocationLimit(intptr_t step) {
     inline_allocation_limit_step_ = step;
     UpdateInlineAllocationLimit(0);
-    top_on_previous_step_ = allocation_info_.top();
+    top_on_previous_step_ = step ? allocation_info_.top() : 0;
   }
 
   // Get the extent of the inactive semispace (for use as a marking stack,