Move focus-navigation tests to WPT

[chromium-wpt-export-bot]

The current state of test_driver.send_keys and content_shell doesn't allow sequential focus navigation [1].

The current plan is to copy the existing focus-navigation tests to WPT at web_tests/external/wpt/shadow-dom/focus-navigation/ and keep the content_shell tests at web_tests/shadow-dom/focus-navigation/ for test coverage.

In the future, once [2] is resolved and we can run WPT test in chrome, we can remove the duplicated tests that depends on the content_shell in web_tests/shadow-dom/focus-navigation/ and enable the web_tests/external/wpt/shadow-dom/focus-navigation/ tests.

[1] https://github.com/whatwg/html/issues/4151 [2] crbug.com/893480

Change-Id: Ib4232494162e19110dfb785034712d56d3ff9491 Bug: 769673, 893480 Reviewed-on: https://chromium-review.googlesource.com/3817589 WPT-Export-Revision: cd2fa81e192651bc79c2b0d14de345e5976d941b