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This adds a cache for both total number of players as well as total
number of events kept by the each individual player, in order to prevent
long-living pages from creating an otherwise unbounded number of events
and creating memory pressure.

There is some logic for selecting which players to prune, but no logic
to do anything special for events yet.

Events, Messages, and Errors are all subject to the same cache size
limits, but the Properties can't grow unbounded so they aren't.

R=dalecurtis

Bug: 1145392
Change-Id: Iab39207276b6179b9f8828df7ced210154ab97dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517928
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827070}