Make sure when we shrink an object that we store a filler first into the free memory before updatin…

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index edb7903cc24582fe4b631bb530a6bdaf232376eb..16a989f85c0dddb0f80b179ef76bde9b46173cff 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1811,6 +1811,14 @@ class HeapObject: public Object {
   // of primitive (non-JS) objects like strings, heap numbers etc.
   inline void set_map_no_write_barrier(Map* value);
 
+  // Get the map using acquire load.
+  inline Map* synchronized_map();
+  inline MapWord synchronized_map_word();
+
+  // Set the map using release store
+  inline void synchronized_set_map(Map* value);
+  inline void synchronized_set_map_word(MapWord map_word);
+
   // During garbage collection, the map word of a heap object does not
   // necessarily contain a map pointer.
   inline MapWord map_word();
@@ -2995,6 +3003,10 @@ class FixedArrayBase: public HeapObject {
   inline int length();
   inline void set_length(int value);
 
+  // Get and set the length using acquire loads and release stores.
+  inline int synchronized_length();
+  inline void synchronized_set_length(int value);
+
   inline static FixedArrayBase* cast(Object* object);
 
   // Layout description.
@@ -8757,6 +8769,11 @@ class String: public Name {
   inline int length();
   inline void set_length(int value);
 
+  // Get and set the length of the string using acquire loads and release
+  // stores.
+  inline int synchronized_length();
+  inline void synchronized_set_length(int value);
+
   // Returns whether this string has only ASCII chars, i.e. all of them can
   // be ASCII encoded.  This might be the case even if the string is
   // two-byte.  Such strings may appear when the embedder prefers