Expose platform thread id on WebThread

Source/platform/WebThread.cpp

Index: Source/platform/WebThread.cpp
diff --git a/Source/platform/WebThread.cpp b/Source/platform/WebThread.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..33a29ab63962034d12e6dc2b54b1345368055b7f
--- /dev/null
+++ b/Source/platform/WebThread.cpp
@@ -0,0 +1,24 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+#include "public/platform/WebThread.h"
+
+#include "wtf/Assertions.h"
+
+#if OS(WIN)
+#include <windows.h>
+#elif OS(POSIX)
+#include <unistd.h>
+#endif
+
+namespace {
+#if OS(WIN)
+COMPILE_ASSERT(sizeof(blink::PlatformThreadId) >= sizeof(DWORD), Size_of_platform_thread_id_is_too_small);
+#elif OS(POSIX)
+COMPILE_ASSERT(sizeof(blink::PlatformThreadId) >= sizeof(pid_t), Size_of_platform_thread_id_is_too_small);
+#else
+#error Unexpected platform
+#endif
+}