Expose platform thread id on WebThread

Source/platform/scheduler/SchedulerTest.cpp

Index: Source/platform/scheduler/SchedulerTest.cpp
diff --git a/Source/platform/scheduler/SchedulerTest.cpp b/Source/platform/scheduler/SchedulerTest.cpp
index 90aa0e203041948309a21feb429677f914c88326..c28cd498a4c26fd3680bb946932e0c14d65848c9 100644
--- a/Source/platform/scheduler/SchedulerTest.cpp
+++ b/Source/platform/scheduler/SchedulerTest.cpp
@@ -38,6 +38,12 @@ public:
         return true;
     }
 
+    virtual blink::PlatformThreadId threadId() const OVERRIDE
+    {
+        ASSERT_NOT_REACHED();
+        return 0;
+    }
+
     virtual void enterRunLoop() OVERRIDE
     {
         ASSERT_NOT_REACHED();