Revert of Delete SignalThread class.

webrtc/base/signalthread.h

Index: webrtc/base/signalthread.h
diff --git a/webrtc/base/signalthread.h b/webrtc/base/signalthread.h
new file mode 100644
index 0000000000000000000000000000000000000000..f5fcf2c4a106b5a9f715c47782552678fa3b2854
--- /dev/null
+++ b/webrtc/base/signalthread.h
@@ -0,0 +1,19 @@
+/*
+ *  Copyright 2004 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_BASE_SIGNALTHREAD_H_
+#define WEBRTC_BASE_SIGNALTHREAD_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/signalthread.h"
+
+#endif  // WEBRTC_BASE_SIGNALTHREAD_H_