Take chrome.sessions to stable

R=kalman@chromium.org,meacer@chromium.org
BUG=123266, 377607

Review URL: https://codereview.chromium.org/338363005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278022 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/_permission_features.json

@@ -861,7 +861,7 @@
     "extension_types": ["platform_app"]
   },
   "sessions": {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app"]
   },
   "signedInDevices": {