Adds a CancellableTaskFactory and exposes postLoadingTasks to Blink

Source/platform/scheduler/Scheduler.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 Scheduler_h
 #define Scheduler_h
 
 #include "platform/PlatformExport.h"
+#include "public/platform/WebThread.h"
 #include "wtf/Functional.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 class TraceLocation;
 class WebScheduler;
 
 // The scheduler is an opinionated gateway for arranging work to be run on the
 // main thread. It decides which tasks get priority over others based on a
 // scheduling policy and the overall system state.
 class PLATFORM_EXPORT Scheduler {
     WTF_MAKE_NONCOPYABLE(Scheduler);
 public:
     // An IdleTask is passed a deadline in CLOCK_MONOTONIC seconds and is expected to complete before this deadline.
     typedef Function<void(double deadlineSeconds)> IdleTask;
 
     static Scheduler* shared();
     static void shutdown();
 
     // For non-critical tasks which may be reordered relative to other task types and may be starved
     // for an arbitrarily long time if no idle time is available.
     void postIdleTask(const TraceLocation&, PassOwnPtr<IdleTask>);
 
+    // For tasks related to loading, e.g. HTML parsing.  Loading tasks usually have default priority
+    // but they may be deprioritized when the user is interacting with the device.
+    // Takes ownership of |WebThread::Task|.
+    void postLoadingTask(const TraceLocation&, WebThread::Task*);
+
     // 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.
     // Must be called on the main thread.
     bool shouldYieldForHighPriorityWork() const;
 
 protected:
     Scheduler(WebScheduler*);
     virtual ~Scheduler();
 
     static Scheduler* s_sharedScheduler;
     WebScheduler* m_webScheduler;
 };
 
 } // namespace blink
 
 #endif // Scheduler_h