GitLab

Adds the ability to record CPU frequency on Android devices, viewable
in the Chrome trace viewer. Records the CPU frequency on a per-cluster
basis, meaning for a Pixel/Pixel 2 we have two separate trace counters
for CPU0-3, and CPU4-7.

The sampler is a TraceLog::EnabledStateObserver, so on Android devices
it will start and stop when tracing is enabled and disabled.

Bug:873303

Change-Id: I1702fa184f16c53f200667528bb88e96957b0e65
Reviewed-on: https://chromium-review.googlesource.com/1123029
Commit-Queue: Troy Hildebrandt <thildebr@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587738}