Fix syntax error in policy_test_cases.json

In policy_test_cases.json there is a typo in the test definition of
`ClearBrowsingDataOnExitList`, `policy_pref_mapping_test` (the suffix
should be `tests`). This typo make to skip this test. This CL fix this
typo making the policy mapping be tested.

Bug: b/178171564
Change-Id: Id432006e15ad7747de1ea0e12c152aca28cb0be6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643722Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Martin Rodriguez <rodmartin@google.com>
Cr-Commit-Position: refs/heads/master@{#846283}

chrome/test/data/policy/policy_test_cases.json

@@ -8342,7 +8342,7 @@
   },
   "ClearBrowsingDataOnExitList": {
     "os": ["win", "linux", "mac", "chromeos"],
-    "policy_pref_mapping_test": [
+    "policy_pref_mapping_tests": [
       {
         "policies": { "ClearBrowsingDataOnExitList": ["browsing_history"] },
         "prefs": { "browser.clear_data.clear_on_exit": { "value": ["browsing_history"]}}