Add platform_thread source files and move types from thread_checker_impl to there.
BUG=
R=henrika@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/52529004
Cr-Commit-Position: refs/heads/master@{#9201}
diff --git a/webrtc/base/platform_thread.h b/webrtc/base/platform_thread.h
new file mode 100644
index 0000000..9629066
--- /dev/null
+++ b/webrtc/base/platform_thread.h
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2015 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_PLATFORM_THREAD_H_
+#define WEBRTC_BASE_PLATFORM_THREAD_H_
+
+#if defined(WEBRTC_WIN)
+#include <winsock2.h>
+#include <windows.h>
+#elif defined(WEBRTC_POSIX)
+#include <pthread.h>
+#include <unistd.h>
+#endif
+
+namespace rtc {
+
+#if defined(WEBRTC_WIN)
+typedef DWORD PlatformThreadId;
+typedef DWORD PlatformThreadRef;
+#elif defined(WEBRTC_POSIX)
+typedef pid_t PlatformThreadId;
+typedef pthread_t PlatformThreadRef;
+#endif
+
+PlatformThreadId CurrentThreadId();
+PlatformThreadRef CurrentThreadRef();
+
+// Compares two thread identifiers for equality.
+bool IsThreadRefEqual(const PlatformThreadRef& a, const PlatformThreadRef& b);
+
+} // namespace rtc
+
+#endif // WEBRTC_BASE_PLATFORM_THREAD_H_