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

ash/wm/window_util.cc

Index: ash/wm/window_util.cc
diff --git a/ash/wm/window_util.cc b/ash/wm/window_util.cc
index 25e84f6db8c2fe3efa1b1a985029fb4a1b27fffe..055244d490b89c6775b64b460ec5e3c37aaa0050 100644
--- a/ash/wm/window_util.cc
+++ b/ash/wm/window_util.cc
@@ -14,6 +14,12 @@
 
 namespace ash {
 
+namespace {
+
+const char kOpenWindowSplitKey[] = "OpenWindowSplit";
+
+}  // namespace
+
 void ActivateWindow(aura::Window* window) {
   aura::client::GetActivationClient()->ActivateWindow(window);
 }
@@ -64,5 +70,13 @@ bool HasFullscreenWindow(const WindowSet& windows) {
   return false;
 }
 
+void SetOpenWindowSplit(aura::Window* window, bool value) {
+  window->SetIntProperty(kOpenWindowSplitKey, value ? 1 : 0);
+}
+
+bool GetOpenWindowSplit(aura::Window* window) {
+  return window->GetIntProperty(kOpenWindowSplitKey) == 1;
+}
+
 }  // namespace window_util
 }  // namespace ash