Introduce --zap-cpp-pointers (off by default)

src/frames.h

Index: src/frames.h
diff --git a/src/frames.h b/src/frames.h
index fb786df539d7b31d6a1a53ad8f6482c411d75973..3171ac9cf6063da8f960e07ab2f13a26c1344786 100644
--- a/src/frames.h
+++ b/src/frames.h
@@ -1606,6 +1606,14 @@ class StackFrameLocator BASE_EMBEDDED {
 // zone memory.
 Vector<StackFrame*> CreateStackMap(Isolate* isolate, Zone* zone);
 
+#ifdef DEBUG
+// Walks the stack, finds sections belonging to C++ frames, looks for values
+// that look like heap pointers inside them, and zaps them. Intended to flush
+// out GC safety issues; only call this when the program must be prepared for
+// GC to happen.
+void ZapHeapPointersInCppFrames(Isolate* isolate, ThreadLocalTop* t = NULL);
+#endif
+
 }  // namespace internal
 }  // namespace v8