Remove more dead code after Object.observe removal

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index ba1e53f8b07e3ad323d92fa82951fe4c4af1d82c..9d450d390da8723019770ba69a0399c1f65d8cd0 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -625,9 +625,7 @@ class Isolate {
   inline Handle<JSGlobalObject> global_object();
 
   // Returns the global proxy object of the current context.
-  JSObject* global_proxy() {
-    return context()->global_proxy();
-  }
+  inline Handle<JSObject> global_proxy();
 
   static int ArchiveSpacePerThread() { return sizeof(ThreadLocalTop); }
   void FreeThreadResources() { thread_local_top_.Free(); }