Reland "Move webrtc/{base => rtc_base}" (https://codereview.webrtc.org/2877023002)

Reland the base->rtc_base without adding stub headers (will be
done in follow-up CL). This preserves git blame history of all files.

BUG=webrtc:7634
NOTRY=True
TBR=kwiberg@webrtc.org

Change-Id: Iea3bb6f3f67b8374c96337b63e8f5aa3e6181012
Reviewed-on: https://chromium-review.googlesource.com/554611
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18821}

webrtc/rtc_base/sequenced_task_checker_impl.h

@@ -0,0 +1,45 @@
+/*
+ *  Copyright (c) 2016 The WebRTC 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 in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_RTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
+#define WEBRTC_RTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
+
+#include "webrtc/base/thread_checker.h"
+
+namespace rtc {
+
+class TaskQueue;
+// Real implementation of SequencedTaskChecker, for use in debug mode, or
+// for temporary use in release mode.
+//
+// Note: You should almost always use the SequencedTaskChecker class to get the
+// right version for your build configuration.
+class SequencedTaskCheckerImpl {
+ public:
+  SequencedTaskCheckerImpl();
+  ~SequencedTaskCheckerImpl();
+
+  bool CalledSequentially() const;
+
+  // Changes the task queue or thread that is checked for in IsCurrent.  This
+  // may be useful when an object may be created on one task queue / thread and
+  // then used exclusively on another thread.
+  void Detach();
+
+ private:
+  typedef const void* QueueId;
+  CriticalSection lock_;
+  ThreadChecker thread_checker_;
+  mutable bool attached_;
+  mutable QueueId valid_queue_;
+};
+
+}  // namespace rtc
+#endif  // WEBRTC_RTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_