[snapshot] serialize embedder-provided external references.

src/external-reference-table.cc

Index: src/external-reference-table.cc
diff --git a/src/external-reference-table.cc b/src/external-reference-table.cc
index 81fce16c64674e988157378f04b677e7f4211796..7e5ef8496e4277c644af71fd9b9728b5bce9f425 100644
--- a/src/external-reference-table.cc
+++ b/src/external-reference-table.cc
@@ -378,6 +378,15 @@ ExternalReferenceTable::ExternalReferenceTable(Isolate* isolate) {
         Deoptimizer::CALCULATE_ENTRY_ADDRESS);
     Add(address, "lazy_deopt");
   }
+
+  // Add external references provided by the embedder (a null-terminated array).
+  intptr_t* api_external_references = isolate->api_external_references();
+  if (api_external_references != nullptr) {
+    while (*api_external_references != 0) {
+      Add(reinterpret_cast<Address>(*api_external_references), "<embedder>");
+      api_external_references++;
+    }
+  }
 }
 
 }  // namespace internal