[snapshot] support multiple contexts in the same snapshot.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index a15a33c60cb95efffb9c3858ff47ce63b2758ce7..ab1708f8fe3ad9b9c153bd9cdd94d5692ef3a361 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -6792,8 +6792,9 @@ class SnapshotCreator {
 
   /**
    * Add a context to be included in the snapshot blob.
+   * \returns the index of the context in the snapshot blob.
    */
-  void AddContext(Local<Context> context);
+  int AddContext(Local<Context> context);

[jochen (gone - plz use gerrit), 2016/06/13 15:24:19]

size_t?

[Yang, 2016/06/14 11:45:30]

This is supposed to mirror the argument of Context::New, and just an index. So I
thought int would be fine.

 
   /**
    * Created a snapshot data blob.
@@ -7097,7 +7098,8 @@ class V8_EXPORT Context {
   static Local<Context> New(
       Isolate* isolate, ExtensionConfiguration* extensions = NULL,
       Local<ObjectTemplate> global_template = Local<ObjectTemplate>(),
-      Local<Value> global_object = Local<Value>());
+      Local<Value> global_object = Local<Value>(),
+      int context_snapshot_index = 0);

[vogelheim, 2016/06/13 16:20:20]

The argument context_snapshot_index has no \param section in the function
comment.

Should default be 0, or some invalid value (-1 or whatnot) for a non-snapshot
context?

[Yang, 2016/06/14 11:45:30]

0 as default reflects the current behavior. To create a context from scratch
(non-snapshot), you would have to set the default snapshot to null.

 
   /**
    * Sets the security token for the context.  To access an object in