heap: allow allocation in gc prologue/epilogue

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 1ac4dfaa0808848947834983bd862aa0fc9f44d2..81c7d4732caa9ad0c5b795ab441e527ec22fb30a 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -2510,6 +2510,8 @@ class Heap {
   bool relocation_mutex_locked_by_optimizer_thread_;
 #endif  // DEBUG;
 
+  int gc_callbacks_depth_;
+
   friend class Factory;
   friend class GCTracer;
   friend class DisallowAllocationFailure;
@@ -2522,6 +2524,7 @@ class Heap {
 #ifdef VERIFY_HEAP
   friend class NoWeakObjectVerificationScope;
 #endif
+  friend class GCCallbacksScope;
 
   DISALLOW_COPY_AND_ASSIGN(Heap);
 };
@@ -2594,6 +2597,18 @@ class NoWeakObjectVerificationScope {
 #endif
 
 
+class GCCallbacksScope {
+ public:
+  explicit inline GCCallbacksScope(Heap* heap);
+  inline ~GCCallbacksScope();
+
+  inline bool CheckReenter();
+
+ private:
+  Heap* heap_;
+};
+
+
 // Visitor class to verify interior pointers in spaces that do not contain
 // or care about intergenerational references. All heap object pointers have to
 // point into the heap to a location that has a map pointer at its first word.