Attach URL for DCHECK in WebURLLoaderMock

...to investigate a DCHECK failure in ClusterFuzz.

Bug: 1027957
Change-Id: I9dbee9026afc93b227a6579552d921077d5aebe6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1952061
Auto-Submit: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721887}

third_party/blink/renderer/platform/testing/weburl_loader_mock.cc

@@ -108,7 +108,7 @@ void WebURLLoaderMock::LoadSynchronously(
 void WebURLLoaderMock::LoadAsynchronously(const WebURLRequest& request,
                                           WebURLLoaderClient* client) {
   DCHECK(client);
-  DCHECK(factory_->IsMockedURL(request.Url()));
+  DCHECK(factory_->IsMockedURL(request.Url())) << request.Url();
   client_ = client;
   factory_->LoadAsynchronouly(request, this);
 }