Initialize ThreadLocalTop.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 01b721dc269048ae134148e21120532c1e7f8dcb..35ffcb4445cf594546b62f12bac74a7afb3025b1 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -185,6 +185,10 @@ class ThreadId {
 
 class ThreadLocalTop BASE_EMBEDDED {
  public:
+  // Does early low-level initialization that does not depend on the
+  // isolate being present.
+  ThreadLocalTop();
+
   // Initialize the thread data.
   void Initialize();
 
@@ -264,6 +268,8 @@ class ThreadLocalTop BASE_EMBEDDED {
   v8::FailedAccessCheckCallback failed_access_check_callback_;
 
  private:
+  void InitializeInternal();
+
   Address try_catch_handler_address_;
 };