Add site_isolation internal repo directory to .gitignore

Bug: 1005895
Change-Id: Ib94a49e4712cf7ff5a5f289f83f653dd8cce7745
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147920Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758964}

.gitignore

@@ -191,6 +191,7 @@ vs-chromium-project.txt
 /components/resources/default_200_percent/google_chrome
 /components/resources/default_300_percent/google_chrome
 /components/search_engines/prepopulated_engines.xml
+/components/site_isolation/internal
 /components/suggestions.xml
 /components/test/data/paint_preview/**/*.pb
 /components/test/data/paint_preview/**/*.skp