[wpt] Run `selection/` with headless shell instead of content shell

See [0] and [1] for background. The test history [2] suggests the
tests stably run as expected on the new runner.

Also, `test_driver.action_sequence()` is fully supported by WebDriver
(unlike content shell's internal APIs), so
`modifying-selection-with-(non-)primary-mouse-button*` is now testable.

[0]: https://groups.google.com/a/chromium.org/g/blink-dev/c/JOu9l2hu1U8
[1]: https://chromium.googlesource.com/chromium/src/+/HEAD/docs/testing/run_web_platform_tests.md
[2]: https://ci.chromium.org/ui/p/chromium/test-search?q=headless_shell_wpt%2Fexternal%2Fwpt%2Fselection%2F

Bug: 40927590
Change-Id: I8783cd179ad560ec85bd1aa8000234aeba1d6edf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5888487
Reviewed-by: Weizhong Xia <weizhong@google.com>
Reviewed-by: Siye Liu <siliu@microsoft.com>
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1360058}