OOR-CORS: Introduce an entry limit to the CORS preflight cache

Since CORS preflight cache can live for a longer time in OOR-CORS mode,
we introduce an entry limit to the CORS preflight cache.
Also this patch adds a metric to count cache entries.

Bug: 736308
Change-Id: Icd0ed0664d4bc592e921a632126ba88d1425f50b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573519
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652458}
4 files changed