Add the IDL and stub implementation for the chrome.autofillPrivate API.

chrome/common/extensions/api/_permission_features.json

Index: chrome/common/extensions/api/_permission_features.json
diff --git a/chrome/common/extensions/api/_permission_features.json b/chrome/common/extensions/api/_permission_features.json
index de15be244324bdc14c4839043c6798deeb81f2d4..e27efa17dcdb557865404ead8343177555817503 100644
--- a/chrome/common/extensions/api/_permission_features.json
+++ b/chrome/common/extensions/api/_permission_features.json
@@ -66,6 +66,12 @@
       ]
     }
   ],
+  "autofillPrivate": {
+    "channel": "trunk",
+    "extension_types": ["extension", "platform_app"],
+    // TODO(khorimoto): Whitelist the settings app once it's an app.
+    "location": "component"
+  },
   "autotestPrivate": {
     "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app"],