Add .gitignore to ShowGlobals directory

Building ShowGlobals causes dirty files to show up in git status. This
is unwanted.

R=stanisc@chromium.org

Review-Url: https://codereview.chromium.org/2649773009
Cr-Original-Commit-Position: refs/heads/master@{#446100}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 24439f5819c904b12f454103bd8c8c73c9d31cc3
1 file changed