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subtle::SpinLock is a spinlock, which has clear shortcomings. Replace it
with base::Lock for the partition lock in PartitionAlloc. As this was
the last user of subtle::SpinLock, remove it as the same time.

Note to perf sheriffs: If there is a regression in range, this is likely
the cause, as the fast path is likely still slower in base::Lock vs
base::subtle::SpinLock.

Bug: 1061437
Change-Id: Idbc4968cfd2fa50bdb1a2468657f4a3da6e29aa4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390066Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804721}