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For each discovery session, report the number of devices
  * discovered,
  * lost, and
  * present when a send starts.

Also, track the number of milliseconds between a the start of a
discovery session and
  * discovery of the first share target,
  * discovery of any share target, and
  * the start of a transfer.

Manually verified using chrome://histograms

Bug: 1105579
Change-Id: Ib998663d968fadcfb016c580ecf9119eefb5f0c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530169Reviewed-by: James Vecore <vecore@google.com>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827054}