Printing API: Change chrome.printntingMetrics channel to stable

Bug: 992218
Change-Id: Ia5548a156040954aba3e902762f97967e211c7b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879907Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Nikita Podguzov <nikitapodguzov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709184}

chrome/common/extensions/api/_permission_features.json

@@ -617,7 +617,7 @@
     "extension_types": ["extension", "legacy_packaged_app"]
   },
   "printingMetrics": {
-    "channel": "dev",
+    "channel": "stable",
     "platforms": ["chromeos"],
     "extension_types": ["extension"],
     "location": "policy"