Adds a CancellableTaskFactory and exposes postLoadingTasks to Blink

Source/platform/scheduler/Scheduler.h

Index: Source/platform/scheduler/Scheduler.h
diff --git a/Source/platform/scheduler/Scheduler.h b/Source/platform/scheduler/Scheduler.h
index 4813b21a53b2dad14c4906ae506c1abda1a3518c..ea7aca1c1526233b7127a80c0672629a53c108cb 100644
--- a/Source/platform/scheduler/Scheduler.h
+++ b/Source/platform/scheduler/Scheduler.h
@@ -30,6 +30,10 @@ public:
     // 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.
+    void postLoadingTask(const TraceLocation&, PassOwnPtr<Closure>);
+
     // 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.