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..8d2f3231d0ab758a4bbb8b3c978758e6e78fe42e 100644
--- a/Source/platform/scheduler/Scheduler.h
+++ b/Source/platform/scheduler/Scheduler.h
@@ -11,6 +11,7 @@
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
+class CancellableTask;

[Sami, 2015/01/26 16:08:15]

Unused?

[alex clarke (OOO till 29th), 2015/01/26 17:00:45]

Done.

 class TraceLocation;
 class WebScheduler;
 
@@ -30,6 +31,10 @@ 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>);

[Sami, 2015/01/26 16:08:15]

Should we have a variant of this that takes a Task? Otherwise
CancellableTaskClosure isn't really compatible with this.

[alex clarke (OOO till 29th), 2015/01/26 17:00:45]

I think WebScheduler::postLoadingTask() takes care of that?

[rmcilroy, 2015/01/26 18:09:01]

Would it be worth only having the WebThread::Task API and exposing ClosureRunner
at the same level as CancelableTaskFactory to enable clients to build Tasks
easily?  This would keep the API consistent.

[alex clarke (OOO till 29th), 2015/01/27 11:41:19]

Done.

+
     // 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.