Output the enumerated files in sorted order when running enumerate_files.py
from gyp, to prevent unnecessary buildfile churn that makes comparing
revisions noisy.

Change-Id: Ia9f759954ea1ae66628f7b6ef3419c47fe65886e
Reviewed-on: https://chromium-review.googlesource.com/187533Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>