Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/ic.cc

Index: src/ic.cc
diff --git a/src/ic.cc b/src/ic.cc
index f01c3d120af1fe6bd00647712ab0e4622b4bb761..2edcb373fe0d04b1a972d7e1d73e91150d7b6f34 100644
--- a/src/ic.cc
+++ b/src/ic.cc
@@ -1864,15 +1864,12 @@ RUNTIME_FUNCTION(MaybeObject*, SharedStoreIC_ExtendStorage) {
 
   FixedArray* new_storage = FixedArray::cast(result);
 
-  Object* to_store = value;
-
   DescriptorArray* descriptors = transition->instance_descriptors();
   PropertyDetails details = descriptors->GetDetails(transition->LastAdded());
-  if (details.representation().IsDouble()) {
-    MaybeObject* maybe_storage =
-        isolate->heap()->AllocateHeapNumber(value->Number());
-    if (!maybe_storage->To(&to_store)) return maybe_storage;
-  }
+  Object* to_store;
+  MaybeObject* maybe_storage = value->AllocateNewStorageFor(
+      isolate->heap(), details.representation());
+  if (!maybe_storage->To(&to_store)) return maybe_storage;
 
   new_storage->set(old_storage->length(), to_store);