chrome_features: Set Usbguard feature flag to disabled by default.

This needs to be enabled by Finch instead of disabled by finch.
This in principle reverts but revision exceptions were blocking the merge:
https://chromium-review.googlesource.com/1176475

BUG=chromium:889118

Change-Id: I401312d32188ff6a0fd97b7fe8838b4371521b95
Reviewed-on: https://chromium-review.googlesource.com/c/1288709
Commit-Queue: Allen Webb <allenwebb@google.com>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600780}

chrome/common/chrome_features.cc

@@ -650,7 +650,7 @@ const base::Feature kMachineLearningService{"MachineLearningService",
 
 // Enable USBGuard at the lockscreen on Chrome OS.
 // TODO(crbug.com/874630): Remove this kill-switch
-const base::Feature kUsbguard{"USBGuard", base::FEATURE_ENABLED_BY_DEFAULT};
+const base::Feature kUsbguard{"USBGuard", base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enable running shill in a minijail sandbox on Chrome OS.
 const base::Feature kShillSandboxing{"ShillSandboxing",