WebUI: Add externs from chrome.mimeHandlerPrivate API.

third_party/closure_compiler/externs/mime_handler_private.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/** @fileoverview Externs generated from namespace: mimeHandlerPrivate */
+
+/** @const */
+chrome.mimeHandlerPrivate = {};
+
+/**
+ * @typedef {?{
+ *   embedded: boolean,
+ *   mimeType: string,
+ *   originalUrl: string,
+ *   responseHeaders: !Object,
+ *   streamUrl: string,
+ *   tabId: number
+ * }}
+ */
+chrome.mimeHandlerPrivate.StreamInfo;
+
+/**
+ * @param {function(!chrome.mimeHandlerPrivate.StreamInfo): void} callback
+ */
+chrome.mimeHandlerPrivate.getStreamInfo = function(callback) {}
+
+/**
+ * @param {function(): void=} opt_callback
+ */
+chrome.mimeHandlerPrivate.abortStream = function(opt_callback) {}