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* Refactors the code to allow adding metrics as part of metrics provider
  interface
* Implements a function in android_metrics_provider to record a new
  histogram which records the current document mode. This is needed for
  better analysis in Hera. I am adding a new histogram because the
  semantics are a bit different from DocumentActivity.RunningMode. The
  previous histogram is used only to record eligible devices that have
  manually opted in or out. The new histogram will record for all devices
  regardless of whether they could possibly be in document mode.

BUG=418642

Review URL: https://codereview.chromium.org/658903002

Cr-Commit-Position: refs/heads/master@{#300375}