Fixed a crash in AutoResumptionHandler

Bug: 931135
Change-Id: I0358a80dfd25ef009a93703663f6170160586e51
Reviewed-on: https://chromium-review.googlesource.com/c/1483359Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634887}

chrome/browser/android/download/download_controller.cc

@@ -382,7 +382,8 @@ void DownloadController::OnDownloadStarted(
   download_item->RemoveObserver(this);
   download_item->AddObserver(this);
 
-  download::AutoResumptionHandler::Get()->OnDownloadStarted(download_item);
+  if (download::AutoResumptionHandler::Get())
+    download::AutoResumptionHandler::Get()->OnDownloadStarted(download_item);
 
   OnDownloadUpdated(download_item);
 }

components/download/public/common/auto_resumption_handler.h

@@ -40,7 +40,8 @@ class COMPONENTS_DOWNLOAD_EXPORT AutoResumptionHandler
       std::unique_ptr<download::TaskManager> task_manager,
       std::unique_ptr<Config> config);
 
-  // Returns the singleton instance of the AutoResumptionHandler.
+  // Returns the singleton instance of the AutoResumptionHandler, or nullptr if
+  // initialization is not yet complete.
   static AutoResumptionHandler* Get();
 
   AutoResumptionHandler(