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.cc

@@ -0,0 +1,63 @@
+/*
+ *  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.
+ */
+
+#include "webrtc/base/sequenced_task_checker_impl.h"
+
+#if defined(WEBRTC_MAC)
+#include <dispatch/dispatch.h>
+#endif
+
+#include "webrtc/base/platform_thread.h"
+#include "webrtc/base/sequenced_task_checker.h"
+#include "webrtc/base/task_queue.h"
+
+namespace rtc {
+
+SequencedTaskCheckerImpl::SequencedTaskCheckerImpl()
+    : attached_(true), valid_queue_(TaskQueue::Current()) {}
+
+SequencedTaskCheckerImpl::~SequencedTaskCheckerImpl() {}
+
+bool SequencedTaskCheckerImpl::CalledSequentially() const {
+  QueueId current_queue = TaskQueue::Current();
+#if defined(WEBRTC_MAC)
+  // If we're not running on a TaskQueue, use the system dispatch queue
+  // label as an identifier.
+  if (current_queue == nullptr)
+    current_queue = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
+#endif
+  CritScope scoped_lock(&lock_);
+  if (!attached_) {  // true if previously detached.
+    valid_queue_ = current_queue;
+    attached_ = true;
+  }
+  if (!valid_queue_)
+    return thread_checker_.CalledOnValidThread();
+  return valid_queue_ == current_queue;
+}
+
+void SequencedTaskCheckerImpl::Detach() {
+  CritScope scoped_lock(&lock_);
+  attached_ = false;
+  valid_queue_ = nullptr;
+  thread_checker_.DetachFromThread();
+}
+
+namespace internal {
+
+SequencedTaskCheckerScope::SequencedTaskCheckerScope(
+    const SequencedTaskChecker* checker) {
+  RTC_DCHECK(checker->CalledSequentially());
+}
+
+SequencedTaskCheckerScope::~SequencedTaskCheckerScope() {}
+
+}  // namespace internal
+}  // namespace rtc