chromeos: Fix display failures when going to mirrored mode.
This fixes an issue where mirrored mode configuration would
fail when the internal and external displays didn't share a
common resolution. A new mode would be created on the
internal display, but it wasn't added to the corresponding
OutputSnapshot struct, resulting in a later failure.
It also moves the mirror-mode-finding code from
RealOutputConfiguratorDelegate to OutputConfigurator so it
can be tested.
BUG=289159
Review URL: https://chromiumcodereview.appspot.com/
24081004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@223642 0039d316-1c4b-4281-b951-d872f2087c98