[snapshot] support multiple contexts in the same snapshot.

src/bootstrapper.h

Index: src/bootstrapper.h
diff --git a/src/bootstrapper.h b/src/bootstrapper.h
index 5563eea043f6fa01715678937ef160b2d63ccd29..be15c61d08f666e85fbab8eb55b70b2c724e6731 100644
--- a/src/bootstrapper.h
+++ b/src/bootstrapper.h
@@ -79,7 +79,7 @@ class Bootstrapper final {
   Handle<Context> CreateEnvironment(
       MaybeHandle<JSGlobalProxy> maybe_global_proxy,
       v8::Local<v8::ObjectTemplate> global_object_template,
-      v8::ExtensionConfiguration* extensions,
+      v8::ExtensionConfiguration* extensions, int context_snapshot_index,
       GlobalContextType context_type = FULL_CONTEXT);
 
   // Detach the environment from its outer global object.