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..c7d252b36d44383cbecc533c4a01e1a10356f2e9 100644
--- a/Source/platform/scheduler/Scheduler.h
+++ b/Source/platform/scheduler/Scheduler.h
@@ -6,6 +6,7 @@
 #define Scheduler_h
 
 #include "platform/PlatformExport.h"
+#include "public/platform/WebThread.h"
 #include "wtf/Functional.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/PassOwnPtr.h"
@@ -30,6 +31,11 @@ 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.
+    // 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.