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This CL fixes a longstanding implementation bug in poisson sampling allocator.
The poisson sampling allocator consistently oversampled allocations,
particularly small ones. The root cause is that muted allocations continue to
affect g_accumulated_bytes_tls. If muted allocations were randomly distributed
with normal allocations, this would have no net effect. However, muted
allocations always happen right after g_accumulated_bytes_tls is reset. This
non-randomness means that muted allocations are almost never sampled, but cause
other allocations to be over-sampled. This CL fixes it by resetting
g_accumulated_bytes_tls back to its original value after we unmute.

This CL also fixes a test bug. To pick up the test-specified sampling rate, we
make several large allocations, flushing the TLS variables which were still
keyed to the old sampling rate.

Bug: 843467
Change-Id: Idea9fab3dfadc014375a3428a7839c76fc635556
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1956266
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723064}