Refactor ChromeContentBrowserClient::GetUserAgentBasedOnPolicy to avoid duplication

[jdapena]

As told in #2 (comment) it would be good to refactor the method ChromeContentBrowserClient::GetUserAgentBasedOnPolicy() from upstream, moving most of its implementation to //components/embedded_support so it can be reused by other embedders as Wolvic Chromium backend.

Then, once we are based on a version of Chromium using that change, update accordingly to use the refactored version.