Add Accessibility Object Model to content features, and add a browser test.

content/public/common/content_features.cc

 // Copyright 2015 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.
 
 #include "build/build_config.h"
 #include "content/public/common/content_features.h"
 
 namespace features {
 
 // All features in alphabetical order.
 
+// Enables the Accessibility Object Model.
+// Explainer: https://github.com/WICG/aom/blob/master/explainer.md
+// Spec: https://wicg.github.io/aom/spec/
+const base::Feature kAccessibilityObjectModel(
+    "AccessibilityObjectModel",
+    base::FEATURE_DISABLED_BY_DEFAULT);
+
 // Enables asm.js to WebAssembly V8 backend.
 // http://asmjs.org/spec/latest/
 const base::Feature kAsmJsToWebAssembly{"AsmJsToWebAssembly",
                                         base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Block subresource requests whose URLs contain embedded credentials (e.g.
 // `https://user:pass@example.com/resource`).
 const base::Feature kBlockCredentialedSubresources{
     "BlockCredentialedSubresources", base::FEATURE_ENABLED_BY_DEFAULT};
 
@@ skipping 271 matching lines @@
 #endif  // !defined(OS_ANDROID)
 
 #if defined(OS_WIN)
 // Emergency "off switch" for new Windows sandbox security mitigation,
 // sandbox::MITIGATION_EXTENSION_POINT_DISABLE.
 const base::Feature kWinSboxDisableExtensionPoints{
     "WinSboxDisableExtensionPoint", base::FEATURE_ENABLED_BY_DEFAULT};
 #endif
 
 }  // namespace features