Enable chrome.networking.config on Stable channel.

BUG=432280

Review URL: https://codereview.chromium.org/1027513005

Cr-Commit-Position: refs/heads/master@{#322048}

extensions/common/api/_permission_features.json

@@ -222,7 +222,7 @@
     "extension_types": ["extension", "legacy_packaged_app", "platform_app"]
   },
   "networking.config": {
-    "channel": "dev",
+    "channel": "stable",
     "platforms": ["chromeos"],
     "extension_types": ["extension"]
   },