Removed a bunch of GetExistingThreadLocal calls by threading the Isolate.

src/handles.h

Index: src/handles.h
diff --git a/src/handles.h b/src/handles.h
index b80dbe523dabe3a684d905a7907b1af233f9d371..3b0d2f7ccf5eb11c262b1f9051ad00eb53c66102 100644
--- a/src/handles.h
+++ b/src/handles.h
@@ -97,8 +97,8 @@ class Handle {
 
 // Convenience wrapper.
 template<class T>
-inline Handle<T> handle(T* t) {
-  return Handle<T>(t);
+inline Handle<T> handle(T* t, Isolate* isolate) {
+  return Handle<T>(t, isolate);
 }