Add comment for background script/page file types

This is a follow-up to https://chromium-review.googlesource.com/957645
in which the reviewer requested that we add some developer documentation
suggesting to use ".js" and ".html" for background script/pages instead
of weird filetypes.

Bug: 809088
Change-Id: Ic1b319522b3fba37b42716d9f5c58bdaf84949f3
Reviewed-on: https://chromium-review.googlesource.com/984992
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Paul Irish <paulirish@chromium.org>
Commit-Queue: Paul Irish <paulirish@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549392}
1 file changed