Fix occasional crash in TryNextRequest

If chrome is shut down, then a user deletes a request from a
notification, the scheduler callback will not be set, and we could
crash trying to inform it.  To fix this, when we start, set an
empty scheduler callback to receive the call, so we don't try
to call through a nullptr.

BUG=64911

Review-Url: https://chromiumcodereview.appspot.com/2429213009
Cr-Commit-Position: refs/heads/master@{#426847}

components/offline_pages/background/request_coordinator.cc

@@ -136,6 +136,7 @@ RequestCoordinator::RequestCoordinator(
       network_quality_estimator_(network_quality_estimator),
       active_request_(nullptr),
       last_offlining_status_(Offliner::RequestStatus::UNKNOWN),
+      scheduler_callback_(base::Bind(&EmptySchedulerCallback)),
       immediate_schedule_callback_(base::Bind(&EmptySchedulerCallback)),
       weak_ptr_factory_(this) {
   DCHECK(policy_ != nullptr);
@@ -239,6 +240,7 @@ bool RequestCoordinator::CancelActiveRequestIfItMatches(
     if (request_ids.end() != std::find(request_ids.begin(), request_ids.end(),
                                        active_request_->request_id())) {
       StopPrerendering(Offliner::RequestStatus::REQUEST_COORDINATOR_CANCELED);
+      active_request_.reset(nullptr);
       return true;
     }
   }
@@ -507,6 +509,7 @@ void RequestCoordinator::TryNextRequest() {
     is_starting_ = false;
 
     // Let the scheduler know we are done processing.
+    // TODO: Make sure the scheduler callback is valid before running it.
     scheduler_callback_.Run(true);
 
     return;