Fix compilation failure on Win.

Review URL: http://codereview.chromium.org/2821016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4939 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.cc

@@ -128,7 +128,7 @@ int Heap::contexts_disposed_ = 0;
 
 int Heap::young_survivors_after_last_gc_ = 0;
 int Heap::high_survival_rate_period_length_ = 0;
-int Heap::survival_rate_ = 0;
+double Heap::survival_rate_ = 0;
 Heap::SurvivalRateTrend Heap::previous_survival_rate_trend_ = Heap::STABLE;
 Heap::SurvivalRateTrend Heap::survival_rate_trend_ = Heap::STABLE;
 bool Heap::bumped_old_gen_limits_ = false;

src/heap.h

@@ -1282,7 +1282,7 @@ class Heap : public AllStatic {
 
   static int young_survivors_after_last_gc_;
   static int high_survival_rate_period_length_;
-  static int survival_rate_;
+  static double survival_rate_;
   static SurvivalRateTrend previous_survival_rate_trend_;
   static SurvivalRateTrend survival_rate_trend_;
   static bool bumped_old_gen_limits_;