Remove XP Condition Variable Affordances

XP and Vista are no longer supported in Chromium.

BUG=579196

Review URL: https://codereview.chromium.org/1860353002

Cr-Commit-Position: refs/heads/master@{#385665}

base/synchronization/condition_variable.h

@@ -75,9 +75,12 @@
 #include <pthread.h>
 #endif
 
+#if defined(OS_WIN)
+#include <windows.h>
+#endif
+
 namespace base {
 
-class ConditionVarImpl;
 class TimeDelta;
 
 class BASE_EXPORT ConditionVariable {
@@ -100,14 +103,15 @@ class BASE_EXPORT ConditionVariable {
  private:
 
 #if defined(OS_WIN)
-  ConditionVarImpl* impl_;
+  CONDITION_VARIABLE cv_;
+  CRITICAL_SECTION* const crit_sec_;
 #elif defined(OS_POSIX)
   pthread_cond_t condition_;
   pthread_mutex_t* user_mutex_;
-#if DCHECK_IS_ON()
-  base::Lock* user_lock_;     // Needed to adjust shadow lock state on wait.
 #endif
 
+#if DCHECK_IS_ON() && (defined(OS_WIN) || defined(OS_POSIX))
+  base::Lock* const user_lock_;  // Needed to adjust shadow lock state on wait.
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(ConditionVariable);