Implement idle task scheduling.

Source/platform/scheduler/TracedTask.h

Index: Source/platform/scheduler/TracedTask.h
diff --git a/Source/platform/scheduler/TracedTask.h b/Source/platform/scheduler/TracedTask.h
index 87f18beec58d5ffefd0d73a249b2ac30771980f6..c21f942b4ca36b139c805c0d10ab90eed8d2c9b2 100644
--- a/Source/platform/scheduler/TracedTask.h
+++ b/Source/platform/scheduler/TracedTask.h
@@ -21,21 +21,50 @@ namespace blink {
 
 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