Pass non-const DownloadTask* to OnDownloadUpdated.

Implementors of DownloadTaskObserver::OnDownloadUpdated should be able
to call RemoveObserver on a ppased task, if the download is complete.

Bug: 780646
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I86f2058e35e9caeb09e32505ef05486f855646eb
Reviewed-on: https://chromium-review.googlesource.com/798312Reviewed-by: Gregory Chatzinoff <gchatz@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520684}

ios/web/download/download_task_impl_unittest.mm

@@ -49,7 +49,7 @@ const char kMimeType[] = "application/pdf";
 
 class MockDownloadTaskObserver : public DownloadTaskObserver {
  public:
-  MOCK_METHOD1(OnDownloadUpdated, void(const DownloadTask* task));
+  MOCK_METHOD1(OnDownloadUpdated, void(DownloadTask* task));
 };
 
 // Mocks DownloadTaskImpl::Delegate's OnTaskUpdated and OnTaskDestroyed methods

ios/web/public/download/download_task_observer.h

@@ -21,7 +21,7 @@ class DownloadTaskObserver {
   // if the download has failed, DownloadTask::GetPercentComplete() to check
   // the download progress, and DownloadTask::GetResponseWriter() to obtain the
   // downloaded data.
-  virtual void OnDownloadUpdated(const DownloadTask* task) {}
+  virtual void OnDownloadUpdated(DownloadTask* task) {}
 
   DownloadTaskObserver() = default;
   virtual ~DownloadTaskObserver() = default;