Added the chooseFileSystemEntries externs

This is an upstream of web_app_file_handling.externs.js to cl/294762966.

Additional context:

chooseFileSystemEntries is still in the draft stage and as such is not
known to closure. This cl adds the function to the native file
handling externs file and gives it a type definition matching the wicg
spec.

https://wicg.github.io/native-file-system/#api-choosefilesystementries

Bug: 1040328,996088
Change-Id: I6f3d2efacc092d36149866f05fccac9cb6e98ada
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084039
Commit-Queue: Zain Afzal <zafzal@google.com>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746673}

chromeos/components/media_app_ui/resources/js/web_app_file_handling.externs.js

@@ -133,5 +133,30 @@ class LaunchQueue {
   setConsumer(consumer) {}
 }
 
+/**
+ * @typedef {{
+ *    description: (string|undefined),
+ *    mimeTypes: (Array<string>|undefined),
+ *    extensions: (Array<string>|undefined)
+ * }}
+ */
+var ChooseFileSystemEntriesOptionsAccepts;
+
+/**
+ * @typedef {{
+ *    type: (string|undefined),
+ *    multiple: (boolean|undefined),
+ *    accepts: (Array<ChooseFileSystemEntriesOptionsAccepts>|undefined),
+ *    excludeAcceptAllOption: (boolean|undefined)
+ * }}
+ */
+var chooseFileSystemEntriesOptions;
+
+/**
+ * @param {(chooseFileSystemEntriesOptions|undefined)} options
+ * @return {Promise<(FileSystemHandle|Array<FileSystemHandle>)>}
+ */
+window.chooseFileSystemEntries = function(options) {};
+
 /** @type {LaunchQueue} */
 window.launchQueue;