Add a new gitignore for media_app_ui/resources/pkg.

This will hold the CIPD package coming in crrev.com/i/2543523

It's not possible to directly replace the git DEPS path without
exploding the build for everyone.

Bug: 1051776
Change-Id: Ibe1f631fd32711aba4c56030da04363f1167d0ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053486Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740968}

.gitignore

@@ -173,6 +173,7 @@ vs-chromium-project.txt
 /chromeos/assistant/internal
 /chromeos/components/help_app_ui/resources/app
 /chromeos/components/media_app_ui/resources/app
+/chromeos/components/media_app_ui/resources/pkg
 /chromeos/profiles/chromeos.orderfile.txt
 /chromeos/profiles/*.local.txt
 /chromeos/profiles/*.afdo.prof