Disable the UI for changing ARC external storage prefs by default

The feature was punted to a later milestone. This CL put the following
features behind the flag:

* 'External storage preferences' submenu in chrome://settings/storage
* "OPEN SETTINGS" button in the Files.app notification.
* "Open settings" link in the Files.app toast.

BUG=936814
TEST=try
TEST=Show chrome://settings/storage, verify the new UI is hidden
TEST=Plug a USB storage device, verify the new UI is hidden in Files.app

Change-Id: I31eb74f673455b83ae0f56f2479a044072156970
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636810
Commit-Queue: Yusuke Sato <[email protected]>
Reviewed-by: Yury Khmel <[email protected]>
Reviewed-by: Luciano Pacheco <[email protected]>
Reviewed-by: Michael Giuffrida <[email protected]>
Cr-Commit-Position: refs/heads/master@{#666531}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index dc9718d5..76aef53 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1681,6 +1681,9 @@
 extern const char kArcUsbHostName[];
 extern const char kArcUsbHostDescription[];
 
+extern const char kArcUsbStorageUIName[];
+extern const char kArcUsbStorageUIDescription[];
+
 extern const char kArcVpnName[];
 extern const char kArcVpnDescription[];