Introduce code flushing of RegExp code.

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 8501)
+++ src/heap.h	(working copy)
@@ -1256,6 +1256,11 @@
     return &external_string_table_;
   }
 
+  // Returns the current sweep generation.
+  int sweep_generation() {
+    return sweep_generation_;
+  }
+
   inline Isolate* isolate();
   bool is_safe_to_read_maps() { return is_safe_to_read_maps_; }
 
@@ -1285,6 +1290,9 @@
   // scavenge since last new space expansion.
   int survived_since_last_expansion_;
 
+  // For keeping track on when to flush RegExp code.
+  int sweep_generation_;
+
   int always_allocate_scope_depth_;
   int linear_allocation_scope_depth_;