GitLab

Acquire the lock only once in ThreadCache::Purge(). Beyond being faster,
this also adds the possibility of amortizing thread cache deallocation
later.

Bug: 998048
Change-Id: I95805fe28f8df557d5783684c172618a304985bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540596
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828219}