message loop: Fix message_pump_kqueue wakeup counter

The thread wakeup count histogram aims at counting wakeups behind
real work, as mentioned in the metric description.

However, on MacOS we currently increment the counter on spurious timer
wakeups as well.

Change the metric to ignore these spurious wakeups.

As spvm@ pointed out, recording the histogram on MacOS shows up in 30%
of browser IO thread samples from the Sampling Profiler. This is
probably happening during idle times, so it should not cause performance
problems.  After this change we expect the time spent on the browser IO
thread to drop to near zero amount, as it is on other platforms.

OBSOLETE_HISTOGRAMS=Patterned histogram Scheduling.MessagePump.WakeupCount.{ThreadName} is replaced by Scheduling.MessagePump.WakeupCount2.{ThreadName}

Bug: 477298286
Change-Id: I29136b99aea858d33ed40836c8f446887f239c53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8270329
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1685028}