Allow stability-checker failures on Travis and report them on Taskcluster (#14096)

Fixes https://github.com/web-platform-tests/wpt/issues/7660.

https://github.com/web-platform-tests/wpt/issues/13194 remains open to
verify that Taskcluster isn't failing to detect flakiness that Travis
would.
diff --git a/.taskcluster.yml b/.taskcluster.yml
index bcac7f5..35aad40 100644
--- a/.taskcluster.yml
+++ b/.taskcluster.yml
@@ -149,12 +149,9 @@
                       ${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};
diff --git a/.travis.yml b/.travis.yml
index d89fada..8d4152f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -51,8 +51,7 @@
         apt:
           packages:
             - libnss3-tools
-      env:
-        - JOB=stability SCRIPT=tools/ci/ci_stability.sh PRODUCT=firefox:nightly
+      env: JOB=stability SCRIPT=tools/ci/ci_stability.sh PRODUCT=firefox:nightly
     - name: "stability (Chrome Dev)"
       if: type = pull_request
       os: linux
@@ -62,8 +61,7 @@
           packages:
             - libappindicator1
             - fonts-liberation
-      env:
-        - JOB=stability SCRIPT=tools/ci/ci_stability.sh PRODUCT=chrome:dev
+      env: JOB=stability SCRIPT=tools/ci/ci_stability.sh PRODUCT=chrome:dev
     - name: "tools/ unittests (Python 2)"
       if: type = pull_request
       os: linux
@@ -103,6 +101,8 @@
     - 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: