Cap the size of the MatchedPropertiesCache.

Currently, the MPC can grow nearly without bound (it is only bounded
by the fact that Oilpan will remove entries when the property sets used
for the key are GC-ed); cap its size to fix this. We implement a simple
LRU scheme, and do the capping in the weak-GC callbacks when we are
iterating over the entire map anyway. The cap is not particularly tuned,
but it does not trigger at all during e.g. a normal Speedometer3 run.

The change is performance-neutral, but we hope to use it to be able
to put more things in the MPC in the future without regressions.

Change-Id: I9f9915067db67d5632d3e8c492be40e3713a5963
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5922862
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1367458}