Move printerProvider API to stable

BUG=488661
TEST=None

Review URL: https://codereview.chromium.org/1128963008

Cr-Commit-Position: refs/heads/master@{#330220}

extensions/common/api/_permission_features.json

@@ -260,7 +260,7 @@
     "extension_types": [ "extension", "legacy_packaged_app", "platform_app" ]
   },
   "printerProvider": {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["extension", "platform_app" ]
   },
   // Note: runtime is not actually a permission, but some systems check these