Adjusting the count requires us to call into Semaphore::WaitFor which
even on a z840 introduces a pause of at least 50us. We often call in
here from the unmapper that tries to add pages. E.g. for reducing the
new space size of 8M we call this for 16x2 pages, resulting in a pause
of 1.6ms for just checking the status of the sweeper tasks.

Avoiding reducing the count reduces the epilogue times. Example: FB
infinite scroll:

Before:
  heap.epilogue
    len: 102
    min: 0.01
    max: 4.83
    avg: 0.140196078431
    [0,5[: 102
After:
  heap.epilogue
    len: 106
    min: 0.01
    max: 0.24
    avg: 0.0260377358491
    [0,5[: 106

BUG=

Change-Id: I296c20ae3ac4b65218e4e038a9dbce504160a764
Reviewed-on: https://chromium-review.googlesource.com/455839
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43863}