Reland "In-object double fields unboxing (for 64-bit only)."

src/string-stream.cc

Index: src/string-stream.cc
diff --git a/src/string-stream.cc b/src/string-stream.cc
index 2101460f1e22d3e886eb23245ab2bbe11a3693f0..21f66a613061d190ad49dca044ecf122a37cef6a 100644
--- a/src/string-stream.cc
+++ b/src/string-stream.cc
@@ -351,8 +351,13 @@ void StringStream::PrintUsingMap(JSObject* js_object) {
         }
         Add(": ");
         FieldIndex index = FieldIndex::ForDescriptor(map, i);
-        Object* value = js_object->RawFastPropertyAt(index);
-        Add("%o\n", value);
+        if (js_object->IsUnboxedDoubleField(index)) {
+          double value = js_object->RawFastDoublePropertyAt(index);
+          Add("<unboxed double> %.16g\n", FmtElm(value));
+        } else {
+          Object* value = js_object->RawFastPropertyAt(index);
+          Add("%o\n", value);
+        }
       }
     }
   }