Add a unique value for ChromeCleanupDistribution in fieldtrial_testing_config

The stable_eset tag was deprecated August 22, but Omaha is still
reporting many users still using the stable_eset tag. It's possible
something is pulling tag values from fieldtrial_testing_config,
so add a unique value for those tags.

Fixed: 1017200
Change-Id: Icc8dd004cf84e6f3af05e03d0eb7a9ce03719617
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876026Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708782}

testing/variations/fieldtrial_testing_config.json

@@ -1535,8 +1535,8 @@
                 {
                     "name": "Stable",
                     "params": {
-                        "cleaner_download_group": "stable/eset",
-                        "reporter_omaha_tag": "stable_eset"
+                        "cleaner_download_group": "fieldtrial",
+                        "reporter_omaha_tag": "fieldtrial"
                     },
                     "enable_features": [
                         "ChromeCleanupDistribution"