Reset percent_complete and received_bytes when DownloadTask is started.

When download is restarted after the failure the progress should be 0%.
This CL fixes the bug where download UI displayed 100% progress after
restarting the failed download.

Bug: 823968
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ia44961f9602ea76b0a731bae1829d48fa5fc533f
Reviewed-on: https://chromium-review.googlesource.com/972575Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544722}

ios/web/download/download_task_impl.mm

@@ -219,6 +219,8 @@ void DownloadTaskImpl::Start(
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   DCHECK_NE(state_, State::kInProgress);
   writer_ = std::move(writer);
+  percent_complete_ = 0;
+  received_bytes_ = 0;
   state_ = State::kInProgress;
   GetCookies(base::Bind(&DownloadTaskImpl::StartWithCookies,
                         weak_factory_.GetWeakPtr()));

ios/web/download/download_task_impl_unittest.mm

@@ -344,10 +344,12 @@ TEST_F(DownloadTaskImplTest, Restarting) {
   ASSERT_TRUE(WaitUntilConditionOrTimeout(kWaitForDownloadTimeout, ^{
     return task_->IsDone();
   }));
+  EXPECT_EQ(100, task_->GetPercentComplete());
 
   // Restart the task.
   EXPECT_CALL(task_observer_, OnDownloadUpdated(task_.get()));
   session_task = Start();
+  EXPECT_EQ(0, task_->GetPercentComplete());
   ASSERT_TRUE(session_task);
   testing::Mock::VerifyAndClearExpectations(&task_observer_);