Remove the removed display from the active list after displays are added

Remove the removed display from the active list after displays are added.

This is necessary because the unified display is needed to
move the windows to new, normal display.

Fix DisplayManagerTestApi::UpdateDisplays so that unified display isn't used for extended mode.

BUG=487368
TEST=covered by test.

Committed: https://crrev.com/01fadba93bdebd1c535f13e56c85722c1fa287cb
Cr-Commit-Position: refs/heads/master@{#329675}

[oshima, 2015-05-13 02:14:46]

oshima@chromium.org changed reviewers:
+ mukai@chromium.org

[Jun Mukai, 2015-05-13 17:47:45]

lgtm

[oshima, 2015-05-13 17:49:06]

The CQ bit was checked by oshima@chromium.org

[commit-bot: I haz the power, 2015-05-13 17:50:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1130653004/20001

[commit-bot: I haz the power, 2015-05-13 17:57:06]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-13 17:57:59]

Patchset 2 (id:??) landed as
https://crrev.com/01fadba93bdebd1c535f13e56c85722c1fa287cb
Cr-Commit-Position: refs/heads/master@{#329675}