[WebLayer] Add note about Network Service running in-process

Bug: 1082230
Change-Id: Iac0fc26a34858508ba1730ed5985c24de56b764b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302989Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789123}

weblayer/browser/system_network_context_manager.h

@@ -52,6 +52,10 @@ class SystemNetworkContextManager {
 
   // Returns a SharedURLLoaderFactory owned by the SystemNetworkContextManager
   // that is backed by the SystemNetworkContext.
+  // NOTE: This factory assumes that the network service is running in the
+  // browser process, which is a valid assumption for Android. If WebLayer is
+  // productionized beyond Android, it will need to be extended to handle
+  // network service crashes.
   scoped_refptr<network::SharedURLLoaderFactory> GetSharedURLLoaderFactory();
 
  private: