Revert r17907 - Make it possible to add more than one piece of embedder data to isolates

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 9dd8e9171d16f3ae39554ddeb8bfef53eacca5b7..339cc327bd9bbf3f9d8c7c1f63e97fb4944c058d 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -92,15 +92,15 @@ class PerIsolateData {
  public:
   explicit PerIsolateData(Isolate* isolate) : isolate_(isolate), realms_(NULL) {
     HandleScope scope(isolate);
-    isolate->SetData(0, this);
+    isolate->SetData(this);
   }
 
   ~PerIsolateData() {
-    isolate_->SetData(0, NULL);  // Not really needed, just to be sure...
+    isolate_->SetData(NULL);  // Not really needed, just to be sure...
   }
 
   inline static PerIsolateData* Get(Isolate* isolate) {
-    return reinterpret_cast<PerIsolateData*>(isolate->GetData(0));
+    return reinterpret_cast<PerIsolateData*>(isolate->GetData());
   }
 
   class RealmScope {