Expose platform thread id on WebThread

Source/platform/scheduler/SchedulerTest.cpp

 // 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 "platform/scheduler/Scheduler.h"
 
 #include "platform/TestingPlatformSupport.h"
 #include "platform/TraceLocation.h"
 #include "public/platform/Platform.h"
@@ skipping 20 matching lines @@
     virtual void postDelayedTask(Task* task, long long delayMs) OVERRIDE
     {
         ASSERT_NOT_REACHED();
     }
 
     virtual bool isCurrentThread() const OVERRIDE
     {
         return true;
     }
 
+    virtual blink::PlatformThreadId threadId() const OVERRIDE
+    {
+        ASSERT_NOT_REACHED();
+        return 0;
+    }
+
     virtual void enterRunLoop() OVERRIDE
     {
         ASSERT_NOT_REACHED();
     }
 
     virtual void exitRunLoop() OVERRIDE
     {
         ASSERT_NOT_REACHED();
     }
 
@@ skipping 337 matching lines @@
     m_platformSupport.triggerSharedTimer();
 
     EXPECT_EQ(1, s_dummyTaskCount);
 
     // Clean up.
     m_scheduler->stopSharedTimer();
     m_scheduler->setSharedTimerFiredFunction(nullptr);
 }
 
 } // namespace