WebUI: Add externs from chrome.mimeHandlerPrivate API.

extensions/common/api/PRESUBMIT.py

Index: extensions/common/api/PRESUBMIT.py
diff --git a/extensions/common/api/PRESUBMIT.py b/extensions/common/api/PRESUBMIT.py
index dbc71cae463b2551d64b76a2580615a868c1bc50..e4fba2539ad4b36dcff4ea4af7a16863783d4aee 100644
--- a/extensions/common/api/PRESUBMIT.py
+++ b/extensions/common/api/PRESUBMIT.py
@@ -29,6 +29,8 @@ def _CheckExterns(input_api, output_api):
     join(api_root, 'bluetooth.idl'): join(externs_root, 'bluetooth.js'),
     join(api_root, 'metrics_private.json'):
         join(externs_root, 'metrics_private.js'),
+    join(api_root, 'mime_handler_private.idl'):
+        join(externs_root, 'mime_handler_private.js'),
     # TODO(rdevlin.cronin): Add more!
   }