Remove the ability to allow multiple gc prologue and gc epilogue callbacks

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
===================================================================
--- runtime/vm/gc_marker.cc	(revision 33620)
+++ runtime/vm/gc_marker.cc	(working copy)
@@ -316,8 +316,8 @@
 
 
 void GCMarker::Prologue(Isolate* isolate, bool invoke_api_callbacks) {
-  if (invoke_api_callbacks) {
-    isolate->gc_prologue_callbacks().Invoke();
+  if (invoke_api_callbacks && (isolate->gc_prologue_callback() != NULL)) {
+    (isolate->gc_prologue_callback())();
   }
   // The store buffers will be rebuilt as part of marking, reset them now.
   isolate->store_buffer()->Reset();
@@ -325,8 +325,8 @@
 
 
 void GCMarker::Epilogue(Isolate* isolate, bool invoke_api_callbacks) {
-  if (invoke_api_callbacks) {
-    isolate->gc_epilogue_callbacks().Invoke();
+  if (invoke_api_callbacks && (isolate->gc_epilogue_callback() != NULL)) {
+    (isolate->gc_epilogue_callback())();
   }
 }