Reland "[chromedriver] Allow clients to stop browser process gracefully"

This is a reland of commit 5b8272d3442389cde1b267296d10c50cd9092dca

Changes since original (PS1):
  Initialize `Capabilities.quit_gracefully` before using (MSAN failure
  log: https://paste.googleplex.com/4895326018994176).

Original change's description:
> [chromedriver] Allow clients to stop browser process gracefully
>
> Add a Chrome-specific capability `quitGracefully` that forces
> chromedriver to attempt to stop the browser with `Browser.close` or
> SIGTERM. wptrunner will enable this capability when used with the `rr`
> debugger (`rr record chrome ...`), which needs time to dump the
> recording. The runner doesn't want to manage `--user-data-dir` or
> `--log-net-log` itself, so the existing code path for graceful quit
> is insufficient.
>
> SIGKILL remains the default behavior so that quitting remains fast for
> clients that don't opt-in.
>
> Bug: 339644807
> Change-Id: I8b4286bb188f36d0c786e78bf6257c516c91f865
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6269180
> Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
> Reviewed-by: Maksim Sadym <sadym@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1427507}

Bug: 339644807
Cq-Include-Trybots: luci.chromium.try:linux-webkit-asan-rel
Cq-Include-Trybots: luci.chromium.try:linux-webkit-msan-rel
Change-Id: Iff6dea47c3d2e3c33844e7117fa61d817d9a56f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6322538
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Maksim Sadym <sadym@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1428384}