Bug 1491924 - Split test-macosx64/debug-jsreftest-e10s across 3 chunks rather than 2, to stop timeouts. r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D14763

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/reftest.yml

@@ -76,7 +76,6 @@ jsreftest:
             linux64-ccov/.*: 5
             linux64-qr/.*: 4
             linux32/debug: 5
-            macosx.*: 2
             macosx64-ccov/debug: 5
             default: 3
     max-run-time: