Remove obsolete "previous" field from Genesis.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 3ef8d37769e272948faf20a5b645bec46e84ea0a..702500549082e12bbd0a9cd8c04b11ac5b9769cd 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -149,8 +149,6 @@ class Genesis BASE_EMBEDDED {
 
   Handle<Context> result() { return result_; }
 
-  Genesis* previous() { return previous_; }
-
   Isolate* isolate() const { return isolate_; }
   Factory* factory() const { return isolate_->factory(); }
   Heap* heap() const { return isolate_->heap(); }
@@ -159,11 +157,6 @@ class Genesis BASE_EMBEDDED {
   Handle<Context> native_context_;
   Isolate* isolate_;
 
-  // There may be more than one active genesis object: When GC is
-  // triggered during environment creation there may be weak handle
-  // processing callbacks which may create new environments.
-  Genesis* previous_;
-
   Handle<Context> native_context() { return native_context_; }
 
   // Creates some basic objects. Used for creating a context from scratch.