Move IdleTask to WebThread side 1/3

public/platform/WebScheduler.h

Index: public/platform/WebScheduler.h
diff --git a/public/platform/WebScheduler.h b/public/platform/WebScheduler.h
index 8214156af70e83abd5c0646a7ed629a55d08d50b..7e900015add361a348b4e25942bd7d2ff029674a 100644
--- a/public/platform/WebScheduler.h
+++ b/public/platform/WebScheduler.h
@@ -18,13 +18,7 @@ class BLINK_PLATFORM_EXPORT WebScheduler {
 public:
     virtual ~WebScheduler() { }
 
-    // An IdleTask is passed a deadline in CLOCK_MONOTONIC seconds and is
-    // expected to complete before this deadline.
-    class IdleTask {
-    public:
-        virtual ~IdleTask() { }
-        virtual void run(double deadlineSeconds) = 0;
-    };
+    typedef WebThread::IdleTask IdleTask;
 
     // Returns true if there is high priority work pending on the main thread
     // and the caller should yield to let the scheduler service that work.