Replace NOTREACHED() with DCHECK(false) (fixes #3500)

Restores the old behavior of assertion in Debug build only.

libcef/browser/alloy/dialogs/alloy_constrained_window_views_client.cc

@@ -29,7 +29,7 @@ class AlloyConstrainedWindowViewsClient
     if (auto browser = GetPreferredBrowser(parent)) {
       return browser->platform_delegate()->GetWebContentsModalDialogHost();
     }
-    NOTREACHED();
+    DCHECK(false);
     return nullptr;
   }
 
@@ -73,4 +73,4 @@ class AlloyConstrainedWindowViewsClient
 std::unique_ptr<constrained_window::ConstrainedWindowViewsClient>
 CreateAlloyConstrainedWindowViewsClient() {
   return std::make_unique<AlloyConstrainedWindowViewsClient>();
-}
+}