[snapshot] support multiple contexts in the same snapshot.

src/snapshot/startup-serializer.cc

Index: src/snapshot/startup-serializer.cc
diff --git a/src/snapshot/startup-serializer.cc b/src/snapshot/startup-serializer.cc
index c882ceb95434d23bd528c8f66a8931334a18b474..573e73d174ddb6e09ef780b1d8e98d3faf2e9952 100644
--- a/src/snapshot/startup-serializer.cc
+++ b/src/snapshot/startup-serializer.cc
@@ -90,6 +90,17 @@ void StartupSerializer::SerializeWeakReferencesAndDeferred() {
   Pad();
 }
 
+int StartupSerializer::PartialSnapshotCacheIndex(HeapObject* heap_object) {
+  int index;
+  if (!partial_cache_index_map_.LookupOrInsert(heap_object, &index)) {
+    // This object is not part of the partial snapshot cache yet. Add it to the
+    // startup snapshot so we can refer to it via partial snapshot index from
+    // the partial snapshot.
+    VisitPointer(reinterpret_cast<Object**>(&heap_object));
+  }
+  return index;
+}
+
 void StartupSerializer::Synchronize(VisitorSynchronization::SyncTag tag) {
   // We expect the builtins tag after builtins have been serialized.
   DCHECK(!serializing_builtins_ || tag == VisitorSynchronization::kBuiltins);