Remove unused byte from Map::instance_sizes field.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 12eba2ba25f4468a172436daf03d85ac8a252f10..2251f9262f8a6d7de64fb23f151bdf8e4c1c3897 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5554,14 +5554,13 @@ class Map: public HeapObject {
   inline int instance_size();
   inline void set_instance_size(int value);
 
+  // Only to clear an unused byte, remove once byte is used.
+  inline void clear_unused();
+
   // Count of properties allocated in the object.
   inline int inobject_properties();
   inline void set_inobject_properties(int value);
 
-  // Count of property fields pre-allocated in the object when first allocated.
-  inline int pre_allocated_property_fields();
-  inline void set_pre_allocated_property_fields(int value);
-
   // Instance type.
   inline InstanceType instance_type();
   inline void set_instance_type(InstanceType value);
@@ -6003,13 +6002,6 @@ class Map: public HeapObject {
   int NumberOfDescribedProperties(DescriptorFlag which = OWN_DESCRIPTORS,
                                   PropertyAttributes filter = NONE);
 
-  // Returns the number of slots allocated for the initial properties
-  // backing storage for instances of this map.
-  int InitialPropertiesLength() {
-    return pre_allocated_property_fields() + unused_property_fields() -
-        inobject_properties();
-  }
-
   DECLARE_CAST(Map)
 
   // Code cache operations.
@@ -6145,9 +6137,9 @@ class Map: public HeapObject {
   static const int kInObjectPropertiesByte = 1;
   static const int kInObjectPropertiesOffset =
       kInstanceSizesOffset + kInObjectPropertiesByte;
-  static const int kPreAllocatedPropertyFieldsByte = 2;
-  static const int kPreAllocatedPropertyFieldsOffset =
-      kInstanceSizesOffset + kPreAllocatedPropertyFieldsByte;
+  // Note there is one byte available for use here.
+  static const int kUnusedByte = 2;
+  static const int kUnusedOffset = kInstanceSizesOffset + kUnusedByte;
   static const int kVisitorIdByte = 3;
   static const int kVisitorIdOffset = kInstanceSizesOffset + kVisitorIdByte;
 
@@ -6165,6 +6157,7 @@ class Map: public HeapObject {
   static const int kInstanceTypeAndBitFieldOffset =
       kInstanceAttributesOffset + 0;
   static const int kBitField2Offset = kInstanceAttributesOffset + 2;
+  static const int kUnusedPropertyFieldsByte = 3;
   static const int kUnusedPropertyFieldsOffset = kInstanceAttributesOffset + 3;
 
   STATIC_ASSERT(kInstanceTypeAndBitFieldOffset ==