Adds a CancellableTaskFactory and exposes postLoadingTasks to Blink

public/platform/WebScheduler.h

Index: public/platform/WebScheduler.h
diff --git a/public/platform/WebScheduler.h b/public/platform/WebScheduler.h
index 49421a8bfb8bb40fa6527bd3fe0c3df4f37f0e47..749f58e4d29f68400813863f2a09b79957911207 100644
--- a/public/platform/WebScheduler.h
+++ b/public/platform/WebScheduler.h
@@ -6,6 +6,7 @@
 #define WebScheduler_h
 
 #include "WebCommon.h"
+#include "public/platform/WebThread.h"
 
 namespace blink {
 
@@ -39,6 +40,12 @@ public:
     // starved for an arbitrarily long time if no idle time is available.
     // Takes ownership of |IdleTask|. Can be called from any thread.
     virtual void postIdleTask(const WebTraceLocation&, IdleTask*) { }
+
+    // Schedule a loading task to be run on the Blink main thread. Loading
+    // tasks usually have the default priority, but may be deprioritised
+    // when the user is interacting with the device.
+    // Takes ownership of |WebThread::Task|. Can be called from any thread.
+    virtual void postLoadingTask(const WebTraceLocation&, WebThread::Task*) { }
 };
 
 } // namespace blink