Make download test use two argument ResponseHeader manipulation calls

We want to get rid of the single argument AddHeader() method as the two
argument AddHeader() and SetHeader() methods are safer and more flexible.

Bug: 1068194
Change-Id: If006fe91be7c37d81fea2088c42a8bdb920938a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150029
Auto-Submit: Matt Menke <mmenke@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759110}

components/download/internal/background_service/in_memory_download_unittest.cc

@@ -216,7 +216,7 @@ TEST_F(InMemoryDownloadTest, RedirectResponseHeaders) {
   // Add some random header.
   auto response_head = network::mojom::URLResponseHead::New();
   response_head->headers = base::MakeRefCounted<net::HttpResponseHeaders>("");
-  response_head->headers->AddHeader("X-Random-Test-Header: 123");
+  response_head->headers->SetHeader("X-Random-Test-Header", "123");
 
   // The size must match for download as stream from SimpleUrlLoader.
   network::URLLoaderCompletionStatus status;