Remove unnecessary extern files

These 2 files "connection.js" and "es6_workaround.js" seem to be to
backport some definitions from ES6 and newer Web APIs not covered by
Closure compiler "back on the day", to be able to compile with these
methods that were already available in our Chromium version.

I've run Closure compiler locally as well as in the bots, so Closure
seems to support these on it current version.

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I48f61c9bf439fb8c438dc0155e00f0db9610d96c
Reviewed-on: https://chromium-review.googlesource.com/1068586Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Luciano Pacheco (SYD) <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561376}

ui/file_manager/externs/connection.js

-// Copyright 2014 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.
-
-/**
- * TODO(hirono): Remove this after the build-in extern supports it.
- * @type {{type:string}}
- */
-Navigator.prototype.connection;

ui/file_manager/externs/es6_workaround.js

-// Copyright 2015 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 Partial definitions for ECMAScript 6. To compile the Files app,
-*     some definitions are defined in this file. They should be removed once
-*     they are ready in closure compiler by default.
-* @externs
-*/
-
-/**
- * @param {string} searchString
- * @param {number=} opt_position
- */
-String.prototype.startsWith = function(searchString, opt_position) {};
-
-/**
- * @param {?function(this:S, T, number, !NodeList<T>): ?} callback
- * @template T,S
- * @return {undefined}
- */
-NodeList.prototype.forEach = function(callback) {};

ui/file_manager/file_manager/background/js/BUILD.gn

@@ -110,7 +110,6 @@ js_library("drive_sync_handler") {
   deps = [
     ":progress_center",
   ]
-  externs_list = [ "../../../externs/connection.js" ]
 }
 
 js_library("duplicate_finder") {

ui/file_manager/file_manager/foreground/js/BUILD.gn

@@ -374,7 +374,6 @@ js_library("import_controller") {
     "../../../externs/background/media_import_handler.js",
     "../../../externs/background/media_scanner.js",
     "../../../externs/command_handler_deps.js",
-    "../../../externs/es6_workaround.js",
   ]
 }