Make stability checks on Travis blocking and on Travis not

Fixes #7660.

#13194 remains open to
verify that Taskcluster isn't failing to detect flakiness that Travis
would.

.taskcluster.yml

@@ -149,12 +149,9 @@ tasks:
                       ${browser.name}
                       ${browser.channel};
                     cd ~/web-platform-tests;
-                    result=0;
                     ./tools/ci/taskcluster-run.py
                       --commit-range HEAD^
                       ${browser.name}
                       --
                       --channel=${browser.channel}
-                      ${operation.extra_args} || result=$?;
-                    echo $result > ../artifacts/run-return-code.txt;
-                    echo "Command exited with code $result (failures are allowed while this task is being vetted)."
+                      ${operation.extra_args};

.travis.yml

@@ -103,6 +103,8 @@ matrix:
     - env:  # exclude empty env from the top-level above
   allow_failures:
     - env: JOB=build_css SCRIPT=css/build-css-testsuites.sh
+    - env: JOB=stability SCRIPT=tools/ci/ci_stability.sh PRODUCT=firefox:nightly
+    - env: JOB=stability SCRIPT=tools/ci/ci_stability.sh PRODUCT=chrome:dev
 script:
   - ./tools/ci/run.sh
 cache: