Early return if DownloadResponseHandler::OnComplete() has been called before

In redirection error cases, we call DownloadResponsehandler::OnComplete().
And when connection is closed, OnComplete() could get called again, causing
an NPE exception.
This CL fixes the issue by early return on the 2nd OnComplete() call.

BUG=890525

Change-Id: I2222a7980d7f53ff885b66b96ab2bdd1d963db75
Reviewed-on: https://chromium-review.googlesource.com/1255882Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595667}

components/download/internal/common/download_response_handler.cc

@@ -74,6 +74,7 @@ DownloadResponseHandler::DownloadResponseHandler(
       download_source_(download_source),
       has_strong_validators_(false),
       is_partial_request_(save_info_->offset > 0),
+      completed_(false),
       abort_reason_(DOWNLOAD_INTERRUPT_REASON_NONE) {
   if (!is_parallel_request) {
     RecordDownloadCountWithSource(UNTHROTTLED_COUNT, download_source);
@@ -201,6 +202,10 @@ void DownloadResponseHandler::OnStartLoadingResponseBody(
 
 void DownloadResponseHandler::OnComplete(
     const network::URLLoaderCompletionStatus& status) {
+  if (completed_)
+    return;
+
+  completed_ = true;
   DownloadInterruptReason reason = HandleRequestCompletionStatus(
       static_cast<net::Error>(status.error_code), has_strong_validators_,
       cert_status_, abort_reason_);

components/download/public/common/download_response_handler.h

@@ -93,6 +93,7 @@ class COMPONENTS_DOWNLOAD_EXPORT DownloadResponseHandler
   bool has_strong_validators_;
   GURL origin_;
   bool is_partial_request_;
+  bool completed_;
 
   // The abort reason if this class decides to block the download.
   DownloadInterruptReason abort_reason_;