[base] Add experiment to not use F_FULLFSYNC.

The goal is to run an experiment to measure its cost.

Bug: 40287434
Change-Id: I6b638a33b02e68f3a31f9318a76782ffe2ede570
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5378992


Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Olivier Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283272}