Reapply/merge derived_sources.gyp useless use of cat (demoggification)

This replaces 2 instances in GYP of
<!(cat <(foo_files_list))
with:
<@(files_list)
...avoiding indirection and a shell out to cat.

Nico's 1-line CL:
Make code slightly less complicated
https://codereview.chromium.org/26980003/
...was clobbered by Kihong's large, long-running CL:
Support partial interface for test support idls
https://codereview.chromium.org/24053003/

Nico noted this in comment:
https://codereview.chromium.org/24053003/#msg50

This CL cleans up and merges Nico's prior demoggification.
http://en.wikipedia.org/wiki/Useless_use_of_cat

Review URL: https://codereview.chromium.org/27873002

git-svn-id: svn://svn.chromium.org/blink/trunk@159971 bbb929c8-8fbe-4397-9dbb-9b2b20218538