[serializer] allow test-serialize/PartialSerializerCustomContext to optimize.

Previously, optimization has been disabled because of an assertion
failure, which requires the context to be linked into the weak native
context list of the heap.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/1777883003

Cr-Commit-Position: refs/heads/master@{#34661}

test/cctest/test-serialize.cc

@@ -517,7 +517,6 @@ static void PartiallySerializeCustomContext(
 
 UNINITIALIZED_TEST(PartialSerializerCustomContext) {
   DisableTurbofan();
-  FLAG_crankshaft = false;
   Vector<const byte> startup_blob;
   Vector<const byte> partial_blob;
   PartiallySerializeCustomContext(&startup_blob, &partial_blob);
@@ -540,6 +539,13 @@ UNINITIALIZED_TEST(PartialSerializerCustomContext) {
                  .ToHandleChecked();
       CHECK(root->IsContext());
       Handle<Context> context = Handle<Context>::cast(root);
+
+      // Add context to the weak native context list
+      context->set(Context::NEXT_CONTEXT_LINK,
+                   isolate->heap()->native_contexts_list(),
+                   UPDATE_WEAK_WRITE_BARRIER);
+      isolate->heap()->set_native_contexts_list(*context);
+
       CHECK(context->global_proxy() == *global_proxy);
       Handle<String> o = isolate->factory()->NewStringFromAsciiChecked("o");
       Handle<JSObject> global_object(context->global_object(), isolate);