moz.build needs sorted lists

[ianb]

This must be a recent change, but I'm getting build errors now, such as this build with an error:

[task 2017-11-21T20:23:13.501Z] 20:23:13     INFO -  The error occurred while processing the following file:
[task 2017-11-21T20:23:13.502Z] 20:23:13     INFO -      /builds/worker/workspace/build/src/browser/extensions/screenshots/moz.build
[task 2017-11-21T20:23:13.502Z] 20:23:13     INFO -  The error was triggered on line 29 of this file:
[task 2017-11-21T20:23:13.502Z] 20:23:13     INFO -      'webextension/sitehelper.js'
[task 2017-11-21T20:23:13.502Z] 20:23:13     INFO -  An error was encountered as part of executing the file itself. The error appears to be the fault of the script.
[task 2017-11-21T20:23:13.502Z] 20:23:13     INFO -  The error as reported by Python is:
[task 2017-11-21T20:23:13.502Z] 20:23:13     INFO -      ['UnsortedError: An attempt was made to add an unsorted sequence to a list. The incoming list is unsorted starting at element 0. We expected "webextension/assertIsBlankDocument.js" but got "webextension/domainFromUrl.js"\n']
[task 2017-11-21T20:23:13.901Z] 20:23:13     INFO -  *** Fix above errors and then restart with\
[task 2017-11-21T20:23:13.901Z] 20:23:13     INFO -                 "/usr/local/bin/gmake -f client.mk build"
[task 2017-11-21T20:23:13.901Z] 20:23:13     INFO -  client.mk:145: recipe for target 'configure' failed

Sorting the lists (manually) in moz.build seems to fix the error. We should update mc_export.py. This is reproducible locally with ./mach build

[ianb]

./mach mozbuild-reference makes a very small reference to lists being sorted. I guess that's some indication...?