MemoryPressureMonitor clients call GetCurrentPressureLevel(), sometimes
frequently, to get the current memory pressure reading. Up until
recently the cached memory pressure level could be very stale, so we
updated it on every call to GetCurrentPressureLevel(). Now that the
memory pressure level is updated regularly, we can just return the
cached value (and avoid the extra syscalls).

R=lgrey@chromium.org,mark@chromium.org
BUG=701408

Review-Url: https://codereview.chromium.org/2743253006
Cr-Commit-Position: refs/heads/master@{#456890}