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}
diff --git a/webrtc/rtc_base/thread_checker_impl.h b/webrtc/rtc_base/thread_checker_impl.h
new file mode 100644
index 0000000..05b1b62
--- /dev/null
+++ b/webrtc/rtc_base/thread_checker_impl.h
@@ -0,0 +1,48 @@
+/*
+ *  Copyright (c) 2014 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.
+ */
+
+// Borrowed from Chromium's src/base/threading/thread_checker_impl.h.
+
+#ifndef WEBRTC_RTC_BASE_THREAD_CHECKER_IMPL_H_
+#define WEBRTC_RTC_BASE_THREAD_CHECKER_IMPL_H_
+
+#include "webrtc/base/criticalsection.h"
+#include "webrtc/base/platform_thread_types.h"
+
+namespace rtc {
+
+// Real implementation of ThreadChecker, for use in debug mode, or
+// for temporary use in release mode (e.g. to RTC_CHECK on a threading issue
+// seen only in the wild).
+//
+// Note: You should almost always use the ThreadChecker class to get the
+// right version for your build configuration.
+class ThreadCheckerImpl {
+ public:
+  ThreadCheckerImpl();
+  ~ThreadCheckerImpl();
+
+  bool CalledOnValidThread() const;
+
+  // Changes the thread that is checked for in CalledOnValidThread.  This may
+  // be useful when an object may be created on one thread and then used
+  // exclusively on another thread.
+  void DetachFromThread();
+
+ private:
+  CriticalSection lock_;
+  // This is mutable so that CalledOnValidThread can set it.
+  // It's guarded by |lock_|.
+  mutable PlatformThreadRef valid_thread_;
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_RTC_BASE_THREAD_CHECKER_IMPL_H_