[ZPS] add feature param to fine-tune maximum size of in-memory ZPS cache

In order to determine the right balance between cache performance and
increased memory usage, it's necessary to experiment with different
maximum cache sizes to determine the optimal cache size.

This CL adds a new feature param which defaults to 10 entries, but is
configurable via experimentation to evaluate the optimal cache size.

Bug: 1351224
Change-Id: I1fe76ca8693165f0b378297da70914ec73dbcec5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3856829
Commit-Queue: Khalid Peer <khalidpeer@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040563}