[snapshot] no longer require immovable objects to be on first page.

It used to be that immortal immovable objects have to be on the first page to
not be moved. This is no longer true since we flag pages wrt whether they are
allowed to move.

R=mlippautz@chromium.org

Change-Id: I5c9c88fa358636df119108e16e871815b126ab27
Reviewed-on: https://chromium-review.googlesource.com/575976Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46777}

src/snapshot/startup-serializer.cc

@@ -84,15 +84,6 @@ void StartupSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
   ObjectSerializer object_serializer(this, obj, &sink_, how_to_code,
                                      where_to_point);
   object_serializer.Serialize();
-
-  if (serializing_immortal_immovables_roots_ &&
-      root_index != RootIndexMap::kInvalidRootIndex) {
-    // Make sure that the immortal immovable root has been included in the first
-    // chunk of its reserved space , so that it is deserialized onto the first
-    // page of its space and stays immortal immovable.
-    SerializerReference ref = reference_map_.Lookup(obj);
-    CHECK(ref.is_back_reference() && ref.chunk_index() == 0);
-  }
 }
 
 void StartupSerializer::SerializeWeakReferencesAndDeferred() {