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PartitionAlloc's thread cache improves performance (up to ~3% on
speedometer 2.0 vs jemalloc) on Android Go, but at a memory cost. Since
periodic purge is still disabled, this commit disables the thread cache
on these configurations to recover.

This CL introduces extended_api.h for PartitionAlloc, which is intended
to become the central place to provide finer-grained control when PA-E
is enabled, such as adjusting thread cache behavior (size, reclaiming
behavior, or disabling it), and other allocator tuning parameters.

As such, even if complete thread cache disabling is intended to be
temporary, having an extended API is not.

Bug: 1167218, 998048
Change-Id: I9c9801e41f1a0ed4dab6b3d96647540356428a50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637640
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846097}