base/parameter_pack.h Document __type_pack_element

Adds a comment documenting __type_pack_element

Change-Id: Id3ee1203a7b63f7e0805e0a6f331b4f33162a724
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525938
Auto-Submit: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641549}
1 file changed