Rename Arc.OOMKills.* and Arc.LowMemoryKiller.* to use Memory prefix

Since these metrics currently represent all of the OOM kills and low
memory kills on the system, use Memory prefix instead of Arc. Also
updates histograms to use functions instead of macros, since they aren't
on a critical path.

BUG=b:165194376
BUG=916757
TEST=Build success and MemoryKillsMonitorTest. Expect UMA results with
 new prefix

Change-Id: Ie4492feada24349a72bc8cec64e32f45ca6dae53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488397


Commit-Queue: Willie Koomson <wvk@google.com>
Reviewed-by: Kuo-Hsin Yang <vovoy@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819921}