Small simplification of #include dependencies.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 9abfce3ea5ea6cc7c3649571ddeeef895f086d30..589abfe55b7cf23df3cf197ecaa45331469a2cf7 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -31,12 +31,14 @@
 #include <math.h>
 
 #include "splay-tree-inl.h"
-#include "zone-inl.h"
-
+#include "v8-counters.h"
 
 namespace v8 {
 namespace internal {
 
+// Forward declarations.
+class ZoneScopeInfo;
+
 // Defines all the roots in Heap.
 #define UNCONDITIONAL_STRONG_ROOT_LIST(V)                                      \
   /* Put the byte array map early.  We need it to be in place by the time   */ \