NetworkService: Destroy URLLoaders when a NetworkContext is destroyed.

content/network/url_loader_impl.h

Index: content/network/url_loader_impl.h
diff --git a/content/network/url_loader_impl.h b/content/network/url_loader_impl.h
index a73b05ada3c5354579b65818ba9c736c4a6932e9..b23f2f6db50faead73b2fa545ee004f6efc5ef5a 100644
--- a/content/network/url_loader_impl.h
+++ b/content/network/url_loader_impl.h
@@ -48,6 +48,9 @@ class CONTENT_EXPORT URLLoaderImpl : public mojom::URLLoader,
   void OnResponseStarted(net::URLRequest* url_request, int net_error) override;
   void OnReadCompleted(net::URLRequest* url_request, int bytes_read) override;
 
+  // Returns a WeakPtr so tests can validate that the object was destroyed.
+  base::WeakPtr<URLLoaderImpl> GetWeakPtrForTests();
+
  private:
   void ReadMore();
   void DidRead(uint32_t num_bytes, bool completed_synchronously);