Move platform abstraction to base library

src/base/platform/mutex.cc

 // Copyright 2013 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.
 
-#include "src/platform/mutex.h"
+#include "src/base/platform/mutex.h"
 
 #include <errno.h>
 
 namespace v8 {
-namespace internal {
+namespace base {
 
 #if V8_OS_POSIX
 
 static V8_INLINE void InitializeNativeHandle(pthread_mutex_t* mutex) {
   int result;
 #if defined(DEBUG)
   // Use an error checking mutex in debug mode.
   pthread_mutexattr_t attr;
   result = pthread_mutexattr_init(&attr);
   ASSERT_EQ(0, result);
@@ skipping 160 matching lines @@
   if (!TryLockNativeHandle(&native_handle_)) {
     return false;
   }
 #ifdef DEBUG
   ASSERT_LE(0, level_);
   level_++;
 #endif
   return true;
 }
 
-} }  // namespace v8::internal
+} }  // namespace v8::base