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This CL exposes the path topology of a display via its DisplaySnapshot
representation. This will later be used during display configuration
fallback strategies.

In addition, this CL adds three UMAs to track (two of them use the
new exposed path topology property):
1) the number of configured external displays
2) the number of configured external displays that are connected via
   an MST.
3) the percentage of configured displays connected via MST (i.e.
   num of external MST displays over total num of external displays).

Bug: b:170745518, 1105919
Test: display_unittests && ozone_unittests
Change-Id: Ida7270e60ff8d5969e1664788e2dcf7831d52c8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527955
Commit-Queue: Gil Dekel <gildekel@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831170}