Move platform abstraction to base library

src/sweeper-thread.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_SWEEPER_THREAD_H_
 #define V8_SWEEPER_THREAD_H_
 
 #include "src/base/atomicops.h"
+#include "src/base/platform/platform.h"
 #include "src/flags.h"
-#include "src/platform.h"
 #include "src/utils.h"
 
 #include "src/spaces.h"
 
 #include "src/heap.h"
 
 namespace v8 {
 namespace internal {
 
-class SweeperThread : public Thread {
+class SweeperThread : public base::Thread {
  public:
   explicit SweeperThread(Isolate* isolate);
   ~SweeperThread() {}
 
   void Run();
   void Stop();
   void StartSweeping();
   void WaitForSweeperThread();
   bool SweepingCompleted();
 
   static int NumberOfThreads(int max_available);
 
  private:
   Isolate* isolate_;
   Heap* heap_;
   MarkCompactCollector* collector_;
-  Semaphore start_sweeping_semaphore_;
-  Semaphore end_sweeping_semaphore_;
-  Semaphore stop_semaphore_;
+  base::Semaphore start_sweeping_semaphore_;
+  base::Semaphore end_sweeping_semaphore_;
+  base::Semaphore stop_semaphore_;
   volatile base::AtomicWord stop_thread_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_SWEEPER_THREAD_H_