power: Add metrics for external brightness operations.

Add histogram metrics recording the results of communication
with external displays via DDC/CI:

  Power.ExternalBrightnessRequestResult
  Power.ExternalBrightnessReadResult
  Power.ExternalBrightnessWriteResult
  Power.ExternalDisplayOpenResult

Also make user-requested adjustments of external displays be
reported via the Power.UserBrightnessAdjustmentsPerSession
metric (formerly only used for internal displays).

BUG=chromium:315371
TEST=added tests; also manually checked via
     chrome://histograms that metrics are reported after
     adjusting an external display's brightness

Change-Id: I8f019ff84842bc33fb004bf34ec1b5b2c0cb1d79
Reviewed-on: https://chromium-review.googlesource.com/189399
Tested-by: Daniel Erat <derat@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>
12 files changed