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It only had two callers and it'll become error prone once
ScopedTaskEnvironment's MOCK_TIME mode always mocks TimeTicks::Now().
TimeDelta based TimedWait()'s will remain real-time-all-the-time but
TimeWaitUntil() could be passed
TimeTicks::Now() + TimeDelta::FromSeconds(1) but wait forever or return
right away because Now() is mocked and is effectively a random value
compared to system time.

Context for MOCK_TIME : https://chromium-review.googlesource.com/c/chromium/src/+/1707870

This also enabled an opportunity to clean up the implementations
to invoke TimeTicks::Now() even less than before. All system calls
for waiting are based on delta. So we always use the initial
|wait_delta| and we only need to lookup Now() if the API can
spuriously return too early and |wait_delta| isn't Max().

R=kylechar@chromium.org

Bug: 905412
Change-Id: Ic73ee7ea286d023d2adee624478ff650f94767a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1708841Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682232}