Improve SeqString::Truncate for latest allocated strings.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 39c19a4e3952f2ee5b2178de25fb7b13bc92ff85..40c7aeb50f3af2bba72ac723eeba360bb55c7ec0 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -2419,11 +2419,6 @@ class NewSpace : public Space {
 
   virtual bool ReserveSpace(int bytes);
 
-  // Resizes a sequential string which must be the most recent thing that was
-  // allocated in new space.
-  template <typename StringType>
-  inline void ShrinkStringAtAllocationBoundary(String* string, int len);
-
 #ifdef VERIFY_HEAP
   // Verify the active semispace.
   virtual void Verify();
@@ -2501,6 +2496,7 @@ class NewSpace : public Space {
   MUST_USE_RESULT MaybeObject* SlowAllocateRaw(int size_in_bytes);
 
   friend class SemiSpaceIterator;
+  friend void SeqString::Truncate(int new_length);
 
  public:
   TRACK_MEMORY("NewSpace")