Enable ContainerAppKiller by default

This feature is already enabled by default on ARC Container R and it
works fine. The finch experiment shows no regression when this feature
is enabled [1].

[1]: https://uma.googleplex.com/p/chrome/variations?sid=dbd066fc1f696f5a4169050bddedde63

Bug: b/277527958, b/299379998
Change-Id: I45dfa01922f0c6008e73b6908e23aecdb70a00b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5348462


Reviewed-by: Satoshi Niwa <niwa@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Commit-Queue: Vovo Yang <vovoy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1269935}