Add c/b/ui/media_router/internal to .gitignore

This src-internal directory contains proprietary icons for the Views
Cast dialog.

Bug: 826393

Change-Id: I5a23e2f9d813f8e5fa39eb0adb4a0177814b6b5c
Reviewed-on: https://chromium-review.googlesource.com/1003094
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549234}

.gitignore

@@ -106,6 +106,7 @@ vs-chromium-project.txt
 /chrome/browser/resources/vr/assets/google_chrome/*.png
 /chrome/browser/resources/vr/assets/google_chrome/*.wav
 /chrome/browser/spellchecker/internal
+/chrome/browser/ui/media_router/internal
 /chrome/browser_tests_run.xml
 /chrome/chrome_run.xml
 /chrome/chrome_user32_delay_imports.xml