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The thread cache can accumulate a bounded but non trivial amount of
memory. Add a periodic (from a a timer) reclaiming mechanism. However,
since the timer should not wake up an otherwise idle process, it stops
unless there were enough allocations on the main thread since the last
purge was called.

Bug: 998048
Change-Id: I7e2bb6126b03a00d9c6723d1105d53fe728f8c7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527172
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833742}