[MD Bookmarks] Enable by default.

chrome/browser/extensions/component_loader_unittest.cc

 // Copyright (c) 2012 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 "chrome/browser/extensions/component_loader.h"
 
 #include <stddef.h>
 
 #include <string>
 
@@ skipping 235 matching lines @@
 
   EXPECT_EQ(default_count + 1, registry->enabled_extensions().size());
 }
 
 TEST_F(ComponentLoaderTest, AddOrReplace) {
   EXPECT_EQ(0u, component_loader_.registered_extensions_count());
   component_loader_.AddDefaultComponentExtensions(false);
   size_t const default_count = component_loader_.registered_extensions_count();
   base::FilePath known_extension = GetBasePath()
       .AppendASCII("override_component_extension");
-  base::FilePath unknow_extension = extension_path_;
+  base::FilePath unknown_extension = extension_path_;
   base::FilePath invalid_extension = GetBasePath()
       .AppendASCII("this_path_does_not_exist");
 
-  // Replace a default component extension.
+  // Replace a WebUI.
   component_loader_.AddOrReplace(known_extension);
-  EXPECT_EQ(default_count,
-            component_loader_.registered_extensions_count());
+  EXPECT_EQ(default_count + 1, component_loader_.registered_extensions_count());

[tsergeant, 2017/06/15 07:23:44]

OWNERS might know more, but I think the idea of this test is not to test the
chrome://bookmarks URL override (as in
chrome/test/data/extensions/browsertest/url_rewrite/bookmarks/manifest.json),
but instead to check that the component loader can replace a built-in component
extension with a separate extension on the fly.

To fix this test, I think you should change
chrome/test/data/extensions/override_component_extension/manifest.json to mimic
a different component extension like the PDF viewer, by copying over the "key"
field from chrome/browser/resources/pdf/manifest.json into this file.

[calamity, 2017/06/19 06:00:29]

Done. I wasn't aware of other overrideable component extensions. This is
definitely better, and I think equivalent to the old test.

 
   // Add a new component extension.
-  component_loader_.AddOrReplace(unknow_extension);
-  EXPECT_EQ(default_count + 1,
-            component_loader_.registered_extensions_count());
+  component_loader_.AddOrReplace(unknown_extension);
+  EXPECT_EQ(default_count + 2, component_loader_.registered_extensions_count());
 
   extension_service_.set_ready(true);
   component_loader_.LoadAll();
   ExtensionRegistry* registry = ExtensionRegistry::Get(&profile_);
 
-  EXPECT_EQ(default_count + 1, registry->enabled_extensions().size());
+  EXPECT_EQ(default_count + 2, registry->enabled_extensions().size());
   EXPECT_EQ(0u, extension_service_.unloaded_count());
 
   // replace loaded component extension.
   component_loader_.AddOrReplace(known_extension);
-  EXPECT_EQ(default_count + 1, registry->enabled_extensions().size());
+  EXPECT_EQ(default_count + 2, registry->enabled_extensions().size());
   EXPECT_EQ(1u, extension_service_.unloaded_count());
 
   // Add an invalid component extension.
   std::string extension_id = component_loader_.AddOrReplace(invalid_extension);
   EXPECT_TRUE(extension_id.empty());
 }
 
 }  // namespace extensions