[snapshot] support multiple contexts in the same snapshot.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 214d72e7c27abd486a994b5f33b7f7149d29071a..46e37fa2e043cc9d6bcc0e48b0ef892e9d14769b 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -537,9 +537,13 @@ bool Debug::Load() {
   // Create the debugger context.
   HandleScope scope(isolate_);
   ExtensionConfiguration no_extensions;
+  // TODO(yangguo): we rely on the fact that first context snapshot is usable
+  //                as debug context. This dependency is gone once we remove
+  //                debug context completely.
+  static const int kFirstContextSnapshotIndex = 0;
   Handle<Context> context = isolate_->bootstrapper()->CreateEnvironment(
       MaybeHandle<JSGlobalProxy>(), v8::Local<ObjectTemplate>(), &no_extensions,
-      DEBUG_CONTEXT);
+      kFirstContextSnapshotIndex, DEBUG_CONTEXT);
 
   // Fail if no context could be created.
   if (context.is_null()) return false;