Merge 174008

Merge 174008
> Temporarily remember the primary root window while replacing the display.
> 
> |root_windows_| map becomes empty while replacing display.
> This CL remembers the primary root window temporarily and
> use it when the display is added.
> 
> This is a bit ugly but less risky to merge to 25 branch.
> For 26, I'll refactor and add "replace" protocol to
> DisplayObserver.
> 
> BUG=166673
> TEST=covered by test
> 
> Review URL: https://chromiumcodereview.appspot.com/11644012

TBR=oshima@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=175369