Revert "Remove CpuMonitor and related, unused, code."

This reverts commit 1a24012680f25440aa1d117373df2af14cdc2fc1.

TBR=tommi@webrtc.org,pthatcher@webrtc.org
BUG=

This breaks
http://build.chromium.org/p/chromium.webrtc.fyi/builders/Linux/builds/20148/steps/compile/logs/stdio

Review URL: https://codereview.webrtc.org/1287913004 .

Cr-Commit-Position: refs/heads/master@{#9730}
7 files changed