Added ui/webui/internal to .gitignore

Bug: 852034
Change-Id: Iac5c70336e3868521b8dbfd81c2238122438b466
Reviewed-on: https://chromium-review.googlesource.com/1144631
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576946}

.gitignore

@@ -273,6 +273,7 @@ vs-chromium-project.txt
 /ui/keyboard/keyboard_mojom_bindings.xml
 /ui/surface/surface.xml
 /ui/surface/surface_gpu_tests.xml
+/ui/webui/internal
 /v8
 /webkit/data
 /webpagereplay_logs/