Enable enterprise.reportingPrivate API on Stable.

Change-Id: I1bbeefa0bb68da658443e65ed1f9d63a510c4ed8
Bug: 826298
Reviewed-on: https://chromium-review.googlesource.com/1141050Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Georges Khalil <georgesak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575854}

chrome/common/extensions/api/_permission_features.json

@@ -308,7 +308,7 @@
     ]
   },
   "enterprise.reportingPrivate": {
-    "channel": "beta",
+    "channel": "stable",
     "extension_types": ["extension"],
     "platforms" : ["win", "mac", "linux"],
     "whitelist" : [