Merge branch 'optimise-job-request' into 'master'

Use cached size when passing artifacts to Runner

See merge request gitlab-org/gitlab-ce!23228

changelogs/unreleased/optimise-job-request.yml

+---
+title: Use cached size when passing artifacts to Runner
+merge_request:
+author:
+type: performance

lib/api/entities.rb

@@ -1142,7 +1142,8 @@ module API
     end
 
     class JobArtifactFile < Grape::Entity
-      expose :filename, :size
+      expose :filename
+      expose :cached_size, as: :size
     end
 
     class JobArtifact < Grape::Entity