Document that Isolate::GetCurrent() must not be called before initialization

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index b44cbb5c2724559fdc68703a533294bb78e123da..96e044596d29a67bc71aa4c746a44b0c2180f1ac 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -486,6 +486,7 @@ class Isolate {
 
   // Returns the isolate inside which the current thread is running.
   INLINE(static Isolate* Current()) {
+    DCHECK(base::NoBarrier_Load(&isolate_key_created_) == 1);
     Isolate* isolate = reinterpret_cast<Isolate*>(
         base::Thread::GetExistingThreadLocal(isolate_key_));
     DCHECK(isolate != NULL);
@@ -493,6 +494,7 @@ class Isolate {
   }
 
   INLINE(static Isolate* UncheckedCurrent()) {
+    DCHECK(base::NoBarrier_Load(&isolate_key_created_) == 1);
     return reinterpret_cast<Isolate*>(
         base::Thread::GetThreadLocal(isolate_key_));
   }
@@ -1177,6 +1179,10 @@ class Isolate {
   // A global counter for all generated Isolates, might overflow.
   static base::Atomic32 isolate_counter_;
 
+#if DEBUG
+  static base::Atomic32 isolate_key_created_;
+#endif
+
   void Deinit();
 
   static void SetIsolateThreadLocals(Isolate* isolate,