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This change adds a field-trial testing config for the experiment
SlowDCTimerInterruptsWin so that it can be enabled on 50% of the Beta
channel.

Background on the SlowDCTimerInterruptsWin experiment:

Windows implements timers (e.g., sleep(50), wait(100)) by polling at
some frequency for any waiting timers that may have finished. This is
called the 'timer-interrupt frequency', and effectively limits the
actual granularity of timers.

Chrome frequently raises Windows' global timer-interrupt frequency to
make timers more responsive. However, raising the timer-interrupt
frequency increases system power consumption, so should be done
sparingly.

Chrome currently sets the following timer-interrupt frequency limits:

Default, not raised:      64 Hz
Raised, on wall power:    1000 Hz (MinTimerIntervalHighResMs = 1)
Raised, on battery power: 250 Hz  (MinTimerIntervalLowResMs = 4)

This experiment halves the raised frequency on battery power:

Raised, on battery power: 125 Hz  (MinTimerIntervalLowResMs = 8)

This level has been reported to result in a ~4% power savings and
minimal negative impact on performance (per crbug.com/927165).

Bug: 927165
Change-Id: I1db7cbacac2697f382d815dc2a48ece905d58871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145761Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759776}