Reset histogram allocator after tests

With some test orderings this failure is seen in base_unittests:

    Check failed: !subtle::NoBarrier_Load(&g_histogram_allocator)

Tests are supposed to reset the histogram allocator when they are done
and this failure indicates a test that did not. This change first adds
an OnTestEnd check to detect the error. That makes it trivially obvious
which test needs a call to GlobalHistogramAllocator::ReleaseForTesting()
and this change also adds that call.

This particular failure should be permanently fixed in base_unittests
now.

Bug: 1204201
Change-Id: I2173049708b4ffbfc96fef99fe5e46f6154ceebf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2948066
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#891707}