Remove usages of Heap::NewSpaceStart and its external reference

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index cc8a9beb49eb5a47ef28a06fd6bfa188d96b2bc5..3eedc1eeedbfb6567e2a6066e41469bdeded6812 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -1335,8 +1335,6 @@ bool NewSpace::SetUp(int reserved_semispace_capacity,
   }
   DCHECK(!from_space_.is_committed());  // No need to use memory yet.
 
-  start_ = chunk_base_;
-
   ResetAllocationInfo();
 
   return true;
@@ -1353,10 +1351,8 @@ void NewSpace::TearDown() {
     promoted_histogram_ = NULL;
   }
 
-  start_ = NULL;
   allocation_info_.Reset(nullptr, nullptr);
 
-
   to_space_.TearDown();
   from_space_.TearDown();