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: Ic644053d0a1cc9c986ae47fbe983221d6a62b50b
Reviewed-on: https://chromium-review.googlesource.com/187690Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>