Files.app: Add private API to check if PiexLoader is enabled or not.

third_party/closure_compiler/externs/file_manager_private.js

Index: third_party/closure_compiler/externs/file_manager_private.js
diff --git a/third_party/closure_compiler/externs/file_manager_private.js b/third_party/closure_compiler/externs/file_manager_private.js
index 614e93b6ddccd106b01cc55dd45c017343bef3ce..d1736ca00eccc345529f9f04e027583ee241978e 100644
--- a/third_party/closure_compiler/externs/file_manager_private.js
+++ b/third_party/closure_compiler/externs/file_manager_private.js
@@ -574,6 +574,12 @@ chrome.fileManagerPrivate.isUMAEnabled = function(callback) {};
  */
 chrome.fileManagerPrivate.setEntryTag = function(entryURL, visibility, key, value, callback) {};
 
+/**
+ * Gets a flag indicating whether PiexLoader is enabled.
+ * @param {function(boolean)} callback
+ */
+chrome.fileManagerPrivate.isPiexLoaderEnabled = function(callback) {};
+
 /** @type {!ChromeEvent} */
 chrome.fileManagerPrivate.onMountCompleted;