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.cc b/webrtc/base/platform_thread.cc
new file mode 100644
index 0000000..d043c11
--- /dev/null
+++ b/webrtc/base/platform_thread.cc
@@ -0,0 +1,57 @@
+/*
+ *  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.
+ */
+
+#include "webrtc/base/platform_thread.h"
+
+#include "webrtc/base/checks.h"
+
+#if defined(WEBRTC_LINUX)
+#include <sys/syscall.h>
+#endif
+
+namespace rtc {
+
+PlatformThreadId CurrentThreadId() {
+  PlatformThreadId ret;
+#if defined(WEBRTC_WIN)
+  ret = GetCurrentThreadId();
+#elif defined(WEBRTC_POSIX)
+#if defined(WEBRTC_MAC) || defined(WEBRTC_IOS)
+  ret = pthread_mach_thread_np(pthread_self());
+#elif defined(WEBRTC_LINUX)
+  ret =  syscall(__NR_gettid);
+#elif defined(WEBRTC_ANDROID)
+  ret = gettid();
+#else
+  // Default implementation for nacl and solaris.
+  ret = reinterpret_cast<pid_t>(pthread_self());
+#endif
+#endif  // defined(WEBRTC_POSIX)
+  DCHECK(ret);
+  return ret;
+}
+
+PlatformThreadRef CurrentThreadRef() {
+#if defined(WEBRTC_WIN)
+  return GetCurrentThreadId();
+#elif defined(WEBRTC_POSIX)
+  return pthread_self();
+#endif
+}
+
+bool IsThreadRefEqual(const PlatformThreadRef& a, const PlatformThreadRef& b) {
+#if defined(WEBRTC_WIN)
+  return a == b;
+#elif defined(WEBRTC_POSIX)
+  return pthread_equal(a, b);
+#endif
+}
+
+}  // namespace rtc