Add a general purpose facility for letting background tasks wait until

components/offline_pages/background/request_coordinator.h

Index: components/offline_pages/background/request_coordinator.h
diff --git a/components/offline_pages/background/request_coordinator.h b/components/offline_pages/background/request_coordinator.h
index a6001a04b6e0d9991bbf1a17ff0dcef48122aa8e..bdad378735ae4d8caff7061a01e071a6b8e4c773 100644
--- a/components/offline_pages/background/request_coordinator.h
+++ b/components/offline_pages/background/request_coordinator.h
@@ -47,7 +47,7 @@ class RequestCoordinator : public KeyedService {
   // Starts processing of one or more queued save page later requests.
   // Returns whether processing was started and that caller should expect
   // a callback. If processing was already active, returns false.
-  bool StartProcessing(const base::Callback<void(bool)>& callback);
+  bool StartProcessing(const base::Callback<void(bool)> callback);
 
   // Stops the current request processing if active. This is a way for
   // caller to abort processing; otherwise, processing will complete on
@@ -93,6 +93,8 @@ class RequestCoordinator : public KeyedService {
   Offliner::RequestStatus last_offlining_status_;
   // Class to choose which request to schedule next
   std::unique_ptr<RequestPicker> picker_;
+  // Calling this returns to the scheduler across the JNI bridge.
+  base::Callback<void(bool)> scheduler_callback_;
   // Allows us to pass a weak pointer to callbacks.
   base::WeakPtrFactory<RequestCoordinator> weak_ptr_factory_;