mus: Introduce high-contrast mode in mus.

services/ui/public/interfaces/accessibility_manager.mojom

Index: services/ui/public/interfaces/accessibility_manager.mojom
diff --git a/services/ui/public/interfaces/accessibility_manager.mojom b/services/ui/public/interfaces/accessibility_manager.mojom
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module ui.mojom;
+
+// Allows controlling accessibility in the UI service. Only one client at a time
+// can connect to AccessibilityManager per user. If a new client connects to it,
+// then any previously connected client for that user is disconnected.
+interface AccessibilityManager {
+  // Enables (or disables) high contrast mode.
+  SetHighContrastMode(bool enabled);
+};