Expose platform thread id on WebThread

public/platform/WebThread.h

Index: public/platform/WebThread.h
diff --git a/public/platform/WebThread.h b/public/platform/WebThread.h
index 6bee10fbfebd929305b7c94031e4bf87e545f79b..60e4731048c75607496dbb4195199fff617afe0d 100644
--- a/public/platform/WebThread.h
+++ b/public/platform/WebThread.h
@@ -26,9 +26,13 @@
 #define WebThread_h
 
 #include "WebCommon.h"
+#include <stdint.h>
 
 namespace blink {
 
+// Always an integer value.
+typedef uintptr_t PlatformThreadId;
+
 // Provides an interface to an embedder-defined thread implementation.
 //
 // Deleting the thread blocks until all pending, non-delayed tasks have been
@@ -55,6 +59,7 @@ public:
     virtual void postDelayedTask(Task*, long long delayMs) = 0;
 
     virtual bool isCurrentThread() const = 0;
+    virtual PlatformThreadId threadId() const { return 0; }
 
     virtual void addTaskObserver(TaskObserver*) { }
     virtual void removeTaskObserver(TaskObserver*) { }