Update gitignore rules for viewer assets.

The current gitignore rule wasn't actually catching all the assets we
place into this directory

Change-Id: I384020ca1d99f07ca1d87c2cd069103835617df0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1141796
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Auto-Submit: Greg Daniel <egdaniel@google.com>