[OfflinePages] Clear busy flag in StopPrerendering() in Request Coordinator.

BUG=641000

Review-Url: https://codereview.chromium.org/2280713002
Cr-Commit-Position: refs/heads/master@{#414492}

components/offline_pages/background/request_coordinator.cc

@@ -129,6 +129,7 @@ void RequestCoordinator::StopPrerendering() {
   if (active_request_) {
     RecordOfflinerResultUMA(active_request_->client_id(),
                             last_offlining_status_);
+    is_busy_ = false;
     active_request_.reset();
   }
 

components/offline_pages/background/request_coordinator_unittest.cc

@@ -607,12 +607,17 @@ TEST_F(RequestCoordinatorTest, StartProcessingThenStopProcessingLater) {
   // Let all the async parts of the start processing pipeline run to completion.
   PumpLoop();
 
+  // Coordinator should now be busy.
+  EXPECT_TRUE(is_busy());
+
   // Now we cancel it while the prerenderer is busy.
   coordinator()->StopProcessing();
 
   // Let the async callbacks in the cancel run.
   PumpLoop();
 
+  EXPECT_FALSE(is_busy());
+
   // OfflinerDoneCallback will not end up getting called with status SAVED,
   // since we cancelled the event before the LoadAndSave completed.
   EXPECT_EQ(Offliner::RequestStatus::REQUEST_COORDINATOR_CANCELED,