This prevents the contents of these files showing up in the
output of `git grep`.  This makes git grep much more useful as
these files are minified into a single line which is not human
readable.

Change-Id: I54047fe32d090570fa70935ce108455a47e4d888
Reviewed-on: https://chromium-review.googlesource.com/1232674Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Sam Clegg <sbc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56049}