Remove dev-channel qualifier for documentScan

BUG=378127
R=kalman@chromium.org,asvitkine@chromium.org

Review URL: https://codereview.chromium.org/1137783005

Cr-Commit-Position: refs/heads/master@{#329598}

extensions/common/api/_permission_features.json

@@ -182,7 +182,7 @@
     }
   ],
   "documentScan": {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["extension", "platform_app"],
     "platforms": ["chromeos"]
   },