SnapshotCreator: start from existing snapshot if we have one

src/snapshot/deserializer.cc

Index: src/snapshot/deserializer.cc
diff --git a/src/snapshot/deserializer.cc b/src/snapshot/deserializer.cc
index 86d20e14c02525faa465004768a84415f1fa927a..a670bf796e3deeb08531a9a7d918b1d1642de18c 100644
--- a/src/snapshot/deserializer.cc
+++ b/src/snapshot/deserializer.cc
@@ -76,6 +76,10 @@ void Deserializer::Initialize(Isolate* isolate) {
   external_reference_table_ = ExternalReferenceTable::instance(isolate);
   CHECK_EQ(magic_number_,
            SerializedData::ComputeMagicNumber(external_reference_table_));
+  // The current isolate must have at least as many API-provided external
+  // references as the to-be-deserialized snapshot expects and refers to.
+  CHECK_LE(num_extra_references_,
+           SerializedData::GetExtraReferences(external_reference_table_));
 }
 
 void Deserializer::Deserialize(Isolate* isolate) {