[url] Highlight common error case in DCHECK message

This can be a confusing error message on its own, and requires some
code spelunking to read the comment that really highlights the likely
issue.

Change-Id: I3b4dd2dafdf5b52e8ec6df9252d80ce446e6b088
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2750484
Auto-Submit: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#861856}

url/url_util.cc

@@ -469,10 +469,10 @@ void DoSchemeModificationPreamble() {
   // the SchemeRegistry has been used.
   //
   // This normally means you're trying to set up a new scheme too late or using
-  // the SchemeRegistry too early in your application's init process. Make sure
-  // that you haven't added any static GURL initializers in tests.
+  // the SchemeRegistry too early in your application's init process.
   DCHECK(!g_scheme_registries_used.load())
-      << "Trying to add a scheme after the lists have been used.";
+      << "Trying to add a scheme after the lists have been used. "
+         "Make sure that you haven't added any static GURL initializers in tests.";
 
   // If this assert triggers, it means you've called Add*Scheme after
   // LockSchemeRegistries has been called (see the header file for