Reworks the workspace code. Here's the new heuristics:

ash/wm/window_util.h

Index: ash/wm/window_util.h
diff --git a/ash/wm/window_util.h b/ash/wm/window_util.h
index e43800721bb6ab262652201e3c35ca61c3917fab..f76b8aaf0572732cce988ffc88e793134d038f31 100644
--- a/ash/wm/window_util.h
+++ b/ash/wm/window_util.h
@@ -49,6 +49,11 @@ ASH_EXPORT void RestoreWindow(aura::Window* window);
 typedef std::set<aura::Window*> WindowSet;
 ASH_EXPORT bool HasFullscreenWindow(const WindowSet& windows);
 
+// Sets whether the window should be open in a split mode. Only applicable when
+// workspaces are used.
+ASH_EXPORT void SetOpenWindowSplit(aura::Window* window, bool value);
+ASH_EXPORT bool GetOpenWindowSplit(aura::Window* window);
+
 }  // namespace window_util
 }  // namespace ash