Revert "[CodeHealth] Remove stale base::Feature ArcFilePickerExperiment"

This reverts commit 719adf65bfa7aa52a88c7cbf68e459b5aae9e73f.

Reason for revert: arc.Drivefs.vm and arc.RemovableMedia.vm tests are failing, blocking brya builders.

Original change's description:
> [CodeHealth] Remove stale base::Feature ArcFilePickerExperiment
>
> It's been enabled by default since M76.
>
> Bug: 356233630
> Change-Id: Iea5ee290458ee97ec6be33a08f3d560639f235a8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5971415
> Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
> Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1382195}

Bug: 356233630
Change-Id: I285f1ddbb0c6046a4db6d02d81c432f4df9349bb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6019664
Owners-Override: Navin Ramsaroop <ramsaroop@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Navin Ramsaroop <ramsaroop@google.com>
Cr-Commit-Position: refs/heads/main@{#1382494}