Reland of "Enable inobject double fields unboxing for 64-bit archs."

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 6d77bcc995693186771607d8c95790a36553fa88..58fdd6e5d05a88c16e74c1149aa2ac0d6c5dc03f 100644
--- a/src/layout-descriptor-inl.h
+++ b/src/layout-descriptor-inl.h
@@ -120,7 +120,12 @@ bool LayoutDescriptor::IsTagged(int field_index) {
 
 
 bool LayoutDescriptor::IsFastPointerLayout() {
-  return IsSmi() && (Smi::cast(this)->value() == 0);
+  return this == FastPointerLayout();
+}
+
+
+bool LayoutDescriptor::IsFastPointerLayout(Object* layout_descriptor) {
+  return layout_descriptor == FastPointerLayout();
 }