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Thread caches can accumulate memory over time, increasing memory
footprint. It is then necessary to Purge() them to avoid this. However
thread caches are meant to be thread-unsafe and not use locking on the
fast path, so they cannot be purged from another thread.

This adds a mechanism to purge all thread caches, by synchronously
purging the current thread, and asking other threads to Purge() at the
next deallocation. This does not solve the problem of a thread sleeping
and never deallocating, which will be addressed later.

Bug: 998048
Change-Id: I82c70be128acf2e0801623e81c353bdbc80231a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2428985
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810650}