Polish the cleaning up url request code for parallel download.

content/browser/download/parallel_download_job.cc

Index: content/browser/download/parallel_download_job.cc
diff --git a/content/browser/download/parallel_download_job.cc b/content/browser/download/parallel_download_job.cc
index b6e5cae52fd879d226e9f364b090c8c2f2d94f26..5f282c3536dc091899a7ad58f328480ef4cc4f8f 100644
--- a/content/browser/download/parallel_download_job.cc
+++ b/content/browser/download/parallel_download_job.cc
@@ -100,8 +100,8 @@ void ParallelDownloadJob::CancelRequestWithOffset(int64_t offset) {
   }
 
   auto it = workers_.find(offset);
-  if (it != workers_.end())
-    it->second->Cancel();
+  DCHECK(it != workers_.end());
+  it->second->Cancel();
 }
 
 void ParallelDownloadJob::BuildParallelRequestAfterDelay() {