[api,heap] Fix external GC callbacks.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 46cac758f08ba3c75a0bc0767554f7bb431ea426..0f5e7893c2f165ba969a96ede6a5693c472503cb 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -6990,7 +6990,8 @@ void Isolate::SetReference(internal::Object** parent,
 void Isolate::AddGCPrologueCallback(GCPrologueCallback callback,
                                     GCType gc_type) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
-  isolate->heap()->AddGCPrologueCallback(callback, gc_type);
+  GCCallback new_callback = reinterpret_cast<GCCallback>(callback);
+  isolate->heap()->AddGCPrologueCallback(new_callback, gc_type);
 }
 
 
@@ -7003,7 +7004,8 @@ void Isolate::RemoveGCPrologueCallback(GCPrologueCallback callback) {
 void Isolate::AddGCEpilogueCallback(GCEpilogueCallback callback,
                                     GCType gc_type) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
-  isolate->heap()->AddGCEpilogueCallback(callback, gc_type);
+  GCCallback new_callback = reinterpret_cast<GCCallback>(callback);
+  isolate->heap()->AddGCEpilogueCallback(new_callback, gc_type);
 }