Reland of [heap] Simplify marking of JSFunction and SFI

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index fc7dcff4549b608c8cadf09b14d0630c99d4ef60..f93ec103b71d76bd232c687cd00aff338f3ceb08 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7543,9 +7543,9 @@
 
   // Layout description.
   // Pointer fields.
-  static const int kNameOffset = HeapObject::kHeaderSize;
-  static const int kCodeOffset = kNameOffset + kPointerSize;
-  static const int kOptimizedCodeMapOffset = kCodeOffset + kPointerSize;
+  static const int kCodeOffset = HeapObject::kHeaderSize;
+  static const int kNameOffset = kCodeOffset + kPointerSize;
+  static const int kOptimizedCodeMapOffset = kNameOffset + kPointerSize;
   static const int kScopeInfoOffset = kOptimizedCodeMapOffset + kPointerSize;
   static const int kOuterScopeInfoOffset = kScopeInfoOffset + kPointerSize;
   static const int kConstructStubOffset = kOuterScopeInfoOffset + kPointerSize;
@@ -7670,9 +7670,12 @@
 
   static const int kAlignedSize = POINTER_SIZE_ALIGN(kSize);
 
+  typedef FixedBodyDescriptor<kCodeOffset,
+                              kLastPointerFieldOffset + kPointerSize, kSize>
+      BodyDescriptor;
   typedef FixedBodyDescriptor<kNameOffset,
-                              kLastPointerFieldOffset + kPointerSize,
-                              kSize> BodyDescriptor;
+                              kLastPointerFieldOffset + kPointerSize, kSize>
+      BodyDescriptorWeakCode;
 
   // Bit positions in start_position_and_type.
   // The source code start position is in the 30 most significant bits of