tast: Add "usbguard" dependency.

This is needed to skip the security.USBGuard test on DUTs that don't
have USBGuard installed.

CQ-DEPEND=Iac17327827f42c5ca60921a9bb0cccea3876f093
BUG=chromium:926222
TEST=security.USBGuard no longer fails when run by the suite.

Change-Id: Ifde63d917fe0afc71968ce2e27785587421fe6d6
Reviewed-on: https://chromium-review.googlesource.com/1464359
Commit-Ready: Allen Webb <[email protected]>
Tested-by: Allen Webb <[email protected]>
Reviewed-by: Dan Erat <[email protected]>
diff --git a/docs/test_dependencies.md b/docs/test_dependencies.md
index a4be6d2..4563f93 100644
--- a/docs/test_dependencies.md
+++ b/docs/test_dependencies.md
@@ -52,6 +52,7 @@
     a convertible device or a tablet device.
 *   `tpm` - A [Trusted Platform Module] chip.
 *   `transparent_hugepage` - [Transparent Hugepage] support in the Linux kernel.
+*   `usbguard` - The ability to allow or block USB devices based on policy.
 *   `vm_host` - The ability to [run virtual machines].
 
 Software features are composed from USE flags. [local_test_runner] lists boolean