Implement idle task scheduling.

Source/platform/scheduler/TracedTask.h

 // 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.
 
 #ifndef TracedTask_h
 #define TracedTask_h
 
 #include "platform/Task.h"
 #include "platform/TraceEvent.h"
 #include "platform/TraceLocation.h"
 
 namespace blink {
 
 #ifdef MANGLE_COMPILES_ON_WIN_OK
 // TODO: Once win version compiles correctly when using TRACE_ID_MANGLE remove
 // the ifdef to always mangle
 #define MANGLE(id) TRACE_ID_MANGLE(id)
 #else
 #define MANGLE(id) (id)
 #endif
 
 class TracedTask {
 public:
+    virtual void run() const = 0;
+    virtual ~TracedTask();
+
+protected:
+    TracedTask(const TraceLocation&, const char* traceName);
+    void endFlowTraceEvent() const;
+
+    TraceLocation m_location;
+    const char* m_traceName;
+
+private:
+    // Declared volatile as it is atomically incremented.
+    static volatile int s_nextFlowTraceID;
+    uint64_t m_flowTraceID;
+};
+
+class TracedStandardTask : public TracedTask {
+public:
     typedef Function<void()> Task;
 
-    void run() const;
+    virtual void run() const;
+    virtual ~TracedStandardTask();
 
 private:
     friend class Scheduler;
-    TracedTask(const Task&, const TraceLocation&, const char* traceName);
 
-    // Declared volatile as it is atomically incremented.
-    static volatile int s_nextFlowTraceID;
+    TracedStandardTask(const Task&, const TraceLocation&, const char* traceName);
 
-    uint64_t m_flowTraceID;
     Task m_task;
-    TraceLocation m_location;
-    const char* m_traceName;
+};
+
+class TracedIdleTask : public TracedTask {
+public:
+    typedef Function<void(double deadlineSeconds)> IdleTask;

[Sami, 2014/10/03 10:39:22]

If we have this, do we need the similar typedef in Scheduler?

[rmcilroy, 2014/10/03 21:41:21]

We do if we want people to be able to postIdleTask to the scheduler with only
Scheduler.h included.  It's the same as the typedef in TracedStandardTask above
which is also in Scheduler.h.

+
+    virtual void run() const;
+    virtual ~TracedIdleTask();
+
+private:
+    friend class Scheduler;

[Sami, 2014/10/03 10:39:22]

We're a friend of the Scheduler and they're a friend of us. This is a little too
much sharing I think :) Can we have it go one way only? Or ideally by adding an
interface for the thing that needs to be shared?

[rmcilroy, 2014/10/03 21:41:21]

Removed TracedIdleTask as a friend of Scheduler and made
Scheduler::currentFrameDeadlineSeconds public.  I think this is OK to be public,
but if you prefer an interface let me know.

+
+    TracedIdleTask(const IdleTask&, const TraceLocation&, const char* traceName);
+
+    IdleTask m_idleTask;
 };
 
 } // namespace blink
 
 #endif // TracedTask_h