[Device Posture] Add IDLHarness test for Device Posture.

This is just a very basic IDL test as full testing will be coming later
(we need to be able to change the posture in the tests).

Bug: 40124716
Change-Id: Ife9ee4efdc5d40cac301e0cc308111b182d4d253
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5354691
Reviewed-by: Raphael Kubo Da Costa <[email protected]>
Reviewed-by: Reilly Grant <[email protected]>
Commit-Queue: Alexis Menard <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1270447}

device-posture/META.yml

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+spec: https://w3c.github.io/device-posture/
+suggested_reviewers:
+  - reillyeon
+  - darktears

device-posture/README.md

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+This directory contains (tentative) tests for the
+[Device Posture](https://w3c.github.io/device-posture/) specification.

device-posture/idlharness.https.window.js

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+// META: script=/resources/WebIDLParser.js
+// META: script=/resources/idlharness.js
+
+// https://w3c.github.io/device-posture/
+
+'use strict';
+
+idl_test(["device-posture"], ["html", "dom", "webidl"], (idl_array) => {
+  idl_array.add_objects({
+    DevicePosture: ["navigator.devicePosture"],
+  });
+});