Emit path to a failing request in DCHECK of WebUIDataSourceImpl::StartDataRequest()

This only affects debug builds.

Turns a console error like

  FATAL:web_ui_data_source_impl.cc(296)] Check failed:
      resource_id != -1 (-1 vs. -1)

into something a like

  FATAL:web_ui_data_source_impl.cc(296)] Check failed:
      resource_id != -1 (-1 vs. -1) for assets/02_west.png

Change-Id: Iac2ffb57ef7fe734b165edabb1cc2ddd00660f14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874748
Auto-Submit: Trent Apted <tapted@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708668}

content/browser/webui/web_ui_data_source_impl.cc

@@ -294,7 +294,7 @@ void WebUIDataSourceImpl::StartDataRequest(
   }
 
   int resource_id = PathToIdrOrDefault(CleanUpPath(path));
-  DCHECK_NE(resource_id, -1);
+  DCHECK_NE(resource_id, -1) << " for " << path;
   scoped_refptr<base::RefCountedMemory> response(
       GetContentClient()->GetDataResourceBytes(resource_id));
   callback.Run(response.get());