Move login windows to/from unified desktop. This is necessary because in Unified desktop mode, the WTF for primary display gets recreated.
A few cleanups:
* Removed DisplayManager::multi_display_mode() and use IsInUnifiedMode() instead. It simply meant that it will use UNIFIED mode when there are two displays, and it's hard to use it correctly.
* Moved software_mirroring_enabled() to private, and changed clients to use SoftwareMirroringEnable() instead.
* Moved SetDefaultMultiDisplayMode to test api
* Removed unused function decl and unnecessary comment.
BUG=365662
TEST=RootWindowControllerTest.MoveWindows_LockWindowsInUnified
Review URL: https://codereview.chromium.org/
1120563002
Cr-Commit-Position: refs/heads/master@{#327842}