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

Will reland in two different commits to preserve git blame history.

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

Change-Id: I550da8525aeb9c5b8f96338fcf1c9714f3dcdab1
Reviewed-on: https://chromium-review.googlesource.com/554610
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18820}
diff --git a/webrtc/base/sequenced_task_checker_impl.h b/webrtc/base/sequenced_task_checker_impl.h
index 4972539..684b1dc 100644
--- a/webrtc/base/sequenced_task_checker_impl.h
+++ b/webrtc/base/sequenced_task_checker_impl.h
@@ -11,9 +11,35 @@
 #ifndef WEBRTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
 #define WEBRTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
 
+#include "webrtc/base/thread_checker.h"
 
-// This header is deprecated and is just left here temporarily during
-// refactoring. See https://bugs.webrtc.org/7634 for more details.
-#include "webrtc/rtc_base/sequenced_task_checker_impl.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_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_