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This CL is a prerequisite CL for supporting mirroring across 3+
displays.
Changes:
1. Clean up old code of mirror mode in display manager and make software
and hardware mirroring more intuitive.
2. Fix a bug that misuses mirroring destination display as source in
display_info_provider_chromeos.cc

Design doc:
go/multi-display-mirroring

BUG=607844

Change-Id: I38a065e2693c1094d9b460dd99ad0b2daabd0215
Reviewed-on: https://chromium-review.googlesource.com/734799Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512867}