Promote FSP capabilities manifest entry to stable.

TEST=Tested manually.
BUG=474146

Review URL: https://codereview.chromium.org/1149443002

Cr-Commit-Position: refs/heads/master@{#330535}

chrome/common/extensions/api/_manifest_features.json

@@ -153,7 +153,7 @@
     }
   ],
   "file_system_provider_capabilities": {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["extension", "platform_app"]
   },
   "homepage_url": {