Reland of Added GetCpuTime to base/ to get total CPU time consumed by process for perf tests.

BUG=webrtc:7095

Review-Url: https://codereview.webrtc.org/2695743003
Cr-Commit-Position: refs/heads/master@{#16665}
Committed: https://chromium.googlesource.com/external/webrtc/+/3ff474b72ba1b34027e8391b1014c2838af445d4

patch from issue 2695743003 at patchset 440001 (http://crrev.com/2695743003#ps440001)

Review-Url: https://codereview.webrtc.org/2706823002
Cr-Original-Commit-Position: refs/heads/master@{#16736}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 531100dc7a9c58d8cbe200cf3a65d1254070861d