Rename OomKillsMonitor to MemoryKillsMonitor and move it from
arc namespace to memory namespace. Now it is responsible for
1. Log OOM kill events by listening to kernel messages (in a dedicated thread).
2. Log low memory kill events when TabManager kills processes (called from UI thread).

It logs those events to
1. Chrome UMA
2. A local file if --memory-kills-log is given

It starts a new monitoring session when a new BrowserProcess is created.

BUG=none
TEST=manual

Review-Url: https://codereview.chromium.org/2527973003
Cr-Commit-Position: refs/heads/master@{#435743}