Reland "Add MemoryCacheStrongRefPruningTuneUp fieldtrial config"

This CL relands the field trial configuration for
MemoryCacheStrongRefPruningTuneUp. The initial launch of this experiment
caused regressions on some non-PGO speedometer3 substories, leading to
its reversion.

Subsequent analysis has shown that the overall impact on non-PGO bots is
neutral. More importantly, this experiment has demonstrated a positive
effect on Speedometer3 with PGO.

Bug: 421055126

Change-Id: I694d8a621fece49190e81d30ebcdd976a5deb57d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6643245
Commit-Queue: Alex Attar <aattar@google.com>
Reviewed-by: Olivier Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1473689}