Simplify implementation of Mutex.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index b47452e421feeb5c61f8f083d908956eae99a046..165a226219a7fce26f1fd942db55a618b388f19c 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -1444,12 +1444,9 @@ class FreeListCategory {
   FreeListCategory() :
       top_(NULL),
       end_(NULL),
-      mutex_(OS::CreateMutex()),
       available_(0) {}
 
-  ~FreeListCategory() {
-    delete mutex_;
-  }
+  ~FreeListCategory() {}
 
   intptr_t Concatenate(FreeListCategory* category);
 
@@ -1476,7 +1473,7 @@ class FreeListCategory {
   int available() const { return available_; }
   void set_available(int available) { available_ = available; }
 
-  Mutex* mutex() { return mutex_; }
+  Mutex* mutex() { return &mutex_; }
 
 #ifdef DEBUG
   intptr_t SumFreeList();
@@ -1486,7 +1483,7 @@ class FreeListCategory {
  private:
   FreeListNode* top_;
   FreeListNode* end_;
-  Mutex* mutex_;
+  Mutex mutex_;
 
   // Total available bytes in all blocks of this free list category.
   int available_;