Reland "LayoutDescriptor should inherit from JSTypedArray"

src/layout-descriptor-inl.h

Index: src/layout-descriptor-inl.h
diff --git a/src/layout-descriptor-inl.h b/src/layout-descriptor-inl.h
index ba76704d5fc1f5c8883838a61de046268129c0d9..e682cd6eeb672874890c3bc0f39d28a6eaf4fb60 100644
--- a/src/layout-descriptor-inl.h
+++ b/src/layout-descriptor-inl.h
@@ -22,7 +22,7 @@ Handle<LayoutDescriptor> LayoutDescriptor::New(Isolate* isolate, int length) {
   }
   length = GetSlowModeBackingStoreLength(length);
   return Handle<LayoutDescriptor>::cast(
-      isolate->factory()->NewFixedTypedArray(length, kExternalUint32Array));
+      isolate->factory()->NewJSTypedArray(UINT32_ELEMENTS, length));
 }
 
 
@@ -48,7 +48,7 @@ bool LayoutDescriptor::GetIndexes(int field_index, int* layout_word_index,
   }
 
   *layout_word_index = field_index / kNumberOfBits;
-  CHECK((!IsSmi() && (*layout_word_index < length())) ||
+  CHECK((!IsSmi() && (*layout_word_index < Smi::cast(length())->value())) ||
         (IsSmi() && (*layout_word_index < 1)));
 
   *layout_bit_index = field_index % kNumberOfBits;
@@ -123,7 +123,20 @@ bool LayoutDescriptor::IsSlowLayout() { return !IsSmi(); }
 
 
 int LayoutDescriptor::capacity() {
-  return IsSlowLayout() ? (length() * kNumberOfBits) : kSmiValueSize;
+  return IsSlowLayout() ? (Smi::cast(length())->value() * kNumberOfBits)
+                        : kSmiValueSize;
+}
+
+
+uint32_t LayoutDescriptor::get_scalar(int index) {
+  DCHECK(IsSlowLayout());
+  return GcSafeElements()->get_scalar(index);
+}
+
+
+void LayoutDescriptor::set(int index, uint32_t value) {
+  DCHECK(IsSlowLayout());
+  GcSafeElements()->set(index, value);
 }
 
 
@@ -214,6 +227,16 @@ LayoutDescriptor* LayoutDescriptor::Initialize(
 }
 
 
+FixedTypedArray<Uint32ArrayTraits>* LayoutDescriptor::GcSafeElements() {
+  MapWord map_word = HeapObject::cast(elements())->map_word();
+  if (map_word.IsForwardingAddress()) {
+    return FixedTypedArray<Uint32ArrayTraits>::cast(
+        map_word.ToForwardingAddress());
+  }
+  return FixedTypedArray<Uint32ArrayTraits>::cast(elements());
+}
+
+
 // InobjectPropertiesHelper is a helper class for querying whether inobject
 // property at offset is Double or not.
 LayoutDescriptorHelper::LayoutDescriptorHelper(Map* map)