clean up gitignore; sort, add chromeos/externals, remove trailing /.

R=mtklein@google.com, epoger@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/240173008

git-svn-id: http://skia.googlecode.com/svn/trunk@14249 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed