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

src/elements-kind.cc

Index: src/elements-kind.cc
diff --git a/src/elements-kind.cc b/src/elements-kind.cc
index 200fea600a7623d9816925fd0421110d638842e3..c669917c42612e8665c9731b1a8b6bddc0c0b9c2 100644
--- a/src/elements-kind.cc
+++ b/src/elements-kind.cc
@@ -54,8 +54,14 @@ int ElementsKindToShiftSize(ElementsKind elements_kind) {
 
 int GetDefaultHeaderSizeForElementsKind(ElementsKind elements_kind) {
   STATIC_ASSERT(FixedArray::kHeaderSize == FixedDoubleArray::kHeaderSize);
-  return IsExternalArrayElementsKind(elements_kind)
-      ? 0 : (FixedArray::kHeaderSize - kHeapObjectTag);
+
+  if (IsExternalArrayElementsKind(elements_kind)) {
+    return 0;
+  } else if (IsFixedTypedArrayElementsKind(elements_kind)) {
+    return FixedTypedArrayBase::kDataOffset - kHeapObjectTag;
+  } else {
+    return FixedArray::kHeaderSize - kHeapObjectTag;
+  }
 }