Mark "u2fDevices" permission stable.

This permission is used by whitelisted apps and extensions to access all
FIDO Alliance Universal 2nd Factor devices without a hard-coded list of
all present and future vendor and product IDs.

BUG=374307

Review URL: https://codereview.chromium.org/601423002

Cr-Commit-Position: refs/heads/master@{#296842}

extensions/common/api/_permission_features.json

@@ -133,7 +133,7 @@
   ],
   "u2fDevices": [
     {
-      "channel": "dev",
+      "channel": "stable",
       "extension_types": ["extension", "platform_app"],
       "whitelist": [
         "496B6890097EB6E19809ADEADD095A8721FBB2E0",  // FIDO U2F APIs