Reland: "Download: Fix an issue that download percentage can move backwards."

On certain Android devices, when disconnecting from WIFI, the network
code will report NETWORK_TIMEOUT before NETWORK_DISCONNECT and trigger
auto download resumption. And we erase the data downloaded by parallel
requests, so the download percentage will go backwards.

Instead, we should keep the slice info when resume with an error.

TBR=dtrainor@chromium.org, qinmin@chromium.org

Bug: 837128
Change-Id: I59ddb7ff57551beb862884febe2de3f3ee9e1359
Reviewed-on: https://chromium-review.googlesource.com/1130349Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573547}

components/download/internal/common/download_item_impl.cc

@@ -1443,17 +1443,14 @@ void DownloadItemImpl::Start(
     // DownloadCreateInfo with an intact DownloadSaveInfo.
     DCHECK(new_create_info.save_info);
 
-    int64_t offset = new_create_info.save_info->offset;
     std::unique_ptr<crypto::SecureHash> hash_state =
         new_create_info.save_info->hash_state
             ? new_create_info.save_info->hash_state->Clone()
             : nullptr;
 
-    destination_info_.received_bytes = offset;
     hash_state_ = std::move(hash_state);
     destination_info_.hash.clear();
     deferred_interrupt_reason_ = new_create_info.result;
-    received_slices_.clear();
     TransitionTo(INTERRUPTED_TARGET_PENDING_INTERNAL);
     DetermineDownloadTarget();
     return;