Update audio_processing module

Corresponds to upstream commit 524e9b043e7e86fd72353b987c9d5f6a1ebf83e1

Update notes:

 * Pull in third party license file

 * Replace .gypi files with BUILD.gn to keep track of what changes
   upstream

 * Bunch of new filse pulled in as dependencies

 * Won't build yet due to changes needed on top of these

webrtc/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_BASE_THREAD_CHECKER_IMPL_H_
+#define WEBRTC_BASE_THREAD_CHECKER_IMPL_H_
+
+#include "webrtc/base/criticalsection.h"
+#include "webrtc/base/platform_thread.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:
+  mutable CriticalSection lock_;
+  // This is mutable so that CalledOnValidThread can set it.
+  // It's guarded by |lock_|.
+  mutable PlatformThreadRef valid_thread_;
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_THREAD_CHECKER_IMPL_H_