Add mojo_shell.trace to .gitignore.
Add mojo_shell.trace to .gitignore.

R=jamesr@chromium.org, jamesr

Review URL: https://codereview.chromium.org/1261463009 .

afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo