By default, chromium will build with the open source chromium assets and branding (is_chrome_branded = false in GN args, see also Chrome vs. Chromium).
The main reason for this is that the Google Chrome logo and related assets is a trademark which we don‘t want to release under Chromium’s open source license.
Therefore, if you want to add a trademarked resource, check it into the internal repository, and pick a resource based on the branding (is_chrome_branded in GN, #if defined(GOOGLE_CHROME_BUILD) in cpp). If possible, check an open source version into Chromium, so the feature continues to work as expected in the open source build.
E.g. //components/resources/default_100_percent/chromium vs //components/resources/default_100_percent/google_chrome.
For strings, it’s ok to check them into the open source repository, but make sure that you refer to the correct product, i.e., check in a version of the string that says “Google Chrome” and a version that says “Chromium”.
E.g. //chrome/app/chromium_strings.grd vs //chrome/app/google_chrome_strings.grd.