Fix MultiSourceMemoryPressureMonitor metrics recording.

Updates the MultiSourceMemoryPressureMonitor's timer to use a callback
to a new function, RecordCurrentPressureLevel, as the previous
implementation bound the MemoryPressureLevel observed at the time at
which the timer was started, meaning the same level was recorded
every time the timer went off.

Bug: 980965
Change-Id: I2bcab3631bd0f0185bcb054bf28e748a35cd72d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785258
Commit-Queue: Ryan Powell <ryanpow@google.com>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693415}

• base/util/memory_pressure/multi_source_memory_pressure_monitor.cc[diff]
• base/util/memory_pressure/multi_source_memory_pressure_monitor.h[diff]