Allow mimeHandlerViewGuestInternal on stable channel.

BUG=462507

Review URL: https://codereview.chromium.org/972893003

Cr-Commit-Position: refs/heads/master@{#319014}

chrome/common/extensions/api/_api_features.json

@@ -528,7 +528,7 @@
   "mimeHandlerViewGuestInternal": {
     "internal": true,
     "contexts": "all",
-    "channel": "dev",
+    "channel": "stable",
     "matches": ["<all_urls>"]
   },
   "musicManagerPrivate": {