fix: explicitly mark unlimited requests as max priority (#23118)

It's done for us implicitly here, but doing it ourselves avoids the DCHECK.
```
if ((load_flags_ & LOAD_IGNORE_LIMITS) != 0)
  SetPriority(MAXIMUM_PRIORITY);
```
Refs: https://source.chromium.org/chromium/chromium/src/+/master:net/url_request/url_request.cc;l=433;drc=0e1e59c69a449885aa58c403e05feb8274a67c3b;bpv=1;bpt=1

shell/browser/net/proxying_url_loader_factory.cc

@@ -810,6 +810,7 @@ void ProxyingURLLoaderFactory::CreateLoaderAndStart(
   network::ResourceRequest request = original_request;
 
   if (ShouldIgnoreConnectionsLimit(request)) {
+    request.priority = net::RequestPriority::MAXIMUM_PRIORITY;
     request.load_flags |= net::LOAD_IGNORE_LIMITS;
   }