[Extensions] Search API for NTP moved to stable channel.

Bug: 1108940
Change-Id: I1ed44c12ebddb2a156b2677afb62add26c9221a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367027Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Solomon Kinard <solomonkinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800257}

chrome/common/extensions/api/_permission_features.json

@@ -737,7 +737,7 @@
     ]
   },
   "search": {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["extension"]
   },
   "sessions": {