Bug 1415365 - Add whitelist entry for MOZ_UPDATE_CHANNEL testing; r=mshal

This line was added to the nightly macosx64 mozconfig in 9312a1903bf4
(bug 1391643). But we didn't catch it because compare-mozconfigs
only analyzes the current platform and we cross compile for MacOS
from Linux.

MozReview-Commit-ID: 35Q2c0mybPi

--HG--
extra : rebase_source : 25707037ca07ec22de283abcfe96c4faea71d55e

browser/config/mozconfigs/whitelist

@@ -42,6 +42,7 @@ whitelist['nightly']['linux64'] += [
 
 whitelist['nightly']['macosx64'] += [
     'if test "${MOZ_UPDATE_CHANNEL}" = "nightly"; then',
+    'if test "${MOZ_UPDATE_CHANNEL}" = "nightly-try"; then',
     'ac_add_options --with-macbundlename-prefix=Firefox',
     'fi',
     'mk_add_options MOZ_MAKE_FLAGS="-j12"',