Get context from frame_tree_node_id in AdblockCnameResolveHostClient

The ResponseCallback can outlive the BraveRequestInfo context and thus
we can't trust it to retrieve the default storage partition. Instead,
let's go back to the original code where we would attempt to retrieve
the WebContents from the frame_tree_node_id, early returning if that
was not possible.

Resolves brave/brave-browser#16062

browser/net/brave_ad_block_tp_network_delegate_helper.cc

@@ -91,9 +91,17 @@ class AdblockCnameResolveHostClient : public network::mojom::ResolveHostClient {
     if (secure_dns_config.mode() == net::SecureDnsMode::kSecure)
       optional_parameters->source = net::HostResolverSource::DNS;
 
+    auto* web_contents =
+        content::WebContents::FromFrameTreeNodeId(ctx->frame_tree_node_id);
+    if (!web_contents) {
+      start_time_ = base::TimeTicks::Now();
+      this->OnComplete(net::ERR_FAILED, net::ResolveErrorInfo(), base::nullopt);
+      return;
+    }
+
     network::mojom::NetworkContext* network_context =
         content::BrowserContext::GetDefaultStoragePartition(
-            ctx->browser_context)
+            web_contents->GetBrowserContext())
             ->GetNetworkContext();
 
     start_time_ = base::TimeTicks::Now();