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/about_flags.cc b/chrome/browser/about_flags.cc
index 17a30ab..3848be2 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2236,6 +2236,9 @@
     {"arc-usb-host", flag_descriptions::kArcUsbHostName,
      flag_descriptions::kArcUsbHostDescription, kOsCrOS,
      FEATURE_VALUE_TYPE(arc::kUsbHostFeature)},
+    {"arc-usb-storage-ui", flag_descriptions::kArcUsbStorageUIName,
+     flag_descriptions::kArcUsbStorageUIDescription, kOsCrOS,
+     FEATURE_VALUE_TYPE(arc::kUsbStorageUIFeature)},
     {"arc-vpn", flag_descriptions::kArcVpnName,
      flag_descriptions::kArcVpnDescription, kOsCrOS,
      FEATURE_VALUE_TYPE(arc::kVpnFeature)},