Bug 1346232 - Use Ubuntu 16.04 docker image for stylo reftest to avoid timeouts. r=jmaher

MozReview-Commit-ID: Jk7SnxEvHx4

--HG--
extra : rebase_source : 957d273b86ab529b23e653163b26509274cee3f8

taskcluster/ci/test/tests.yml

@@ -1051,6 +1051,7 @@ reftest-stylo:
     description: "Reftest run for Stylo"
     suite: reftest/reftest-stylo
     treeherder-symbol: tc-R(Rs)
+    docker-image: {"in-tree": "desktop1604-test"}
     chunks: 16
     run-on-projects:
         by-test-platform: