[snapshot] support multiple contexts in the same snapshot.

src/snapshot/startup-serializer.h

Index: src/snapshot/startup-serializer.h
diff --git a/src/snapshot/startup-serializer.h b/src/snapshot/startup-serializer.h
index cf40e8dc4c8845802fb0c6899524a008c337530e..cc66f71ae9e3260660a2196b89ef527b6f2222f2 100644
--- a/src/snapshot/startup-serializer.h
+++ b/src/snapshot/startup-serializer.h
@@ -27,7 +27,34 @@ class StartupSerializer : public Serializer {
   void SerializeStrongReferences();
   void SerializeWeakReferencesAndDeferred();
 
+  int PartialSnapshotCacheIndex(HeapObject* o);
+
  private:
+  class PartialCacheIndexMap : public AddressMapBase {
+   public:
+    PartialCacheIndexMap()
+        : map_(base::HashMap::PointersMatch), next_index_(0) {}
+
+    // Lookup object in the map. Return its index if found, or create
+    // a new entry with new_index as value, and return kInvalidIndex.
+    bool LookupOrInsert(HeapObject* obj, int* index_out) {
+      base::HashMap::Entry* entry = LookupEntry(&map_, obj, false);
+      if (entry != NULL) {
+        *index_out = GetValue(entry);
+        return true;
+      }
+      *index_out = next_index_;
+      SetValue(LookupEntry(&map_, obj, true), next_index_++);
+      return false;
+    }
+
+   private:
+    base::HashMap map_;
+    int next_index_;
+
+    DISALLOW_COPY_AND_ASSIGN(PartialCacheIndexMap);
+  };
+
   // The StartupSerializer has to serialize the root array, which is slightly
   // different.
   void VisitPointers(Object** start, Object** end) override;
@@ -45,6 +72,7 @@ class StartupSerializer : public Serializer {
   bool serializing_builtins_;
   bool serializing_immortal_immovables_roots_;
   std::bitset<Heap::kStrongRootListLength> root_has_been_serialized_;
+  PartialCacheIndexMap partial_cache_index_map_;
   DISALLOW_COPY_AND_ASSIGN(StartupSerializer);
 };