"Set" rather than "Add" Fetch Metadata headers.

This shouldn't have any practical effect, but it looks like there are
cases in which we're calling into `AddAdditionalRequestHeaders` twice
for a single request. We don't have a reduction for this case, but it
seems prudent to fix the problem Once And For All by just making this
technical change.

Bug: 948176
Change-Id: If8d5afde0034c9fe9b2bcc85299b3d15259b27f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1554341Reviewed-by: Eric Lawrence <elawrence@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648469}

third_party/blink/renderer/core/loader/base_fetch_context.cc

@@ -173,11 +173,11 @@ void BaseFetchContext::AddAdditionalRequestHeaders(ResourceRequest& request) {
         }
       }
 
-      request.AddHttpHeaderField("Sec-Fetch-Dest", destination_value);
-      request.AddHttpHeaderField(
+      request.SetHttpHeaderField("Sec-Fetch-Dest", destination_value);
+      request.SetHttpHeaderField(
           "Sec-Fetch-Mode",
           FetchRequestModeToString(request.GetFetchRequestMode()));
-      request.AddHttpHeaderField("Sec-Fetch-Site", site_value);
+      request.SetHttpHeaderField("Sec-Fetch-Site", site_value);
       // We don't set `Sec-Fetch-User` for subresource requests.
     }
   }