[heap] User safer root set accessor when possible.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index ce731cfd5fc81722f4e15a1100793153072c5e11..6a7ddee7e75430a9895044534cd00ebd61f34845 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1246,6 +1246,9 @@ class Heap {
 #undef SYMBOL_ACCESSOR
 
   Object* root(RootListIndex index) { return roots_[index]; }
+  Handle<Object> root_handle(RootListIndex index) {
+    return Handle<Object>(&roots_[index]);
+  }
 
   // Generated code can embed this address to get access to the roots.
   Object** roots_array_start() { return roots_; }