Allow dynamic enabling/disabling of unified desktop

ash/display/display_util.h

Index: ash/display/display_util.h
diff --git a/ash/display/display_util.h b/ash/display/display_util.h
index dd0784bd8e6f8f32fda09ecd4c48faff396a1b9d..0e4dadf97d0cc0e39277c88ec0b71e1485295778 100644
--- a/ash/display/display_util.h
+++ b/ash/display/display_util.h
@@ -87,11 +87,15 @@ ASH_EXPORT int FindDisplayIndexContainingPoint(
     const std::vector<gfx::Display>& displays,
     const gfx::Point& point_in_screen);
 
-// Creates the DisplayIdPair where ids are sorted in the following manner.
-// 1) ID for internal display comes first.
-// 2) If none of the ids are internal, sorted by the output index.
+// Creates the DisplayIdPair where ids are sorted using |CompareDisplayIds|
+// below.
 ASH_EXPORT DisplayIdPair CreateDisplayIdPair(int64 id1, int64 id2);
 
+// Returns true if one of following conditinos is met.
+// 1) id1 is internal.
+// 2) output index of id1 < output index of id2 and id2 isn't internal.
+ASH_EXPORT bool CompareDisplayIds(int64 id1, int64 id2);
+
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_DISPLAY_UTIL_H_