Introduce a base pointer field in FixedTypedArrayBase and teach GC about it

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 79a8fbafbe8769104feef8626efb7ff82b8db7ff..7b9796481eab87e4a6602069ce7161662834f2b5 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4593,8 +4593,21 @@ class ExternalFloat64Array: public ExternalArray {
 
 class FixedTypedArrayBase: public FixedArrayBase {
  public:
+  // [base_pointer]: For now, points to the FixedTypedArrayBase itself.
+  DECL_ACCESSORS(base_pointer, Object)
+
+  // Dispatched behavior.
+  inline void FixedTypedArrayBaseIterateBody(ObjectVisitor* v);
+
+  template <typename StaticVisitor>
+  inline void FixedTypedArrayBaseIterateBody();
+
   DECLARE_CAST(FixedTypedArrayBase)
 
+  static const int kBasePointerOffset =
+      FixedArrayBase::kHeaderSize + kPointerSize;
+  static const int kHeaderSize = kBasePointerOffset + kPointerSize;
+
   static const int kDataOffset = DOUBLE_POINTER_ALIGN(kHeaderSize);
 
   inline int size();