athena: Allow getting into split-view mode from overview-mode.

athena/wm/split_view_controller.cc

Index: athena/wm/split_view_controller.cc
diff --git a/athena/wm/split_view_controller.cc b/athena/wm/split_view_controller.cc
index 595a61ae8e0870903d53effa6929c81a828f96bc..3d0cdcb84a5b98af3afe24dc23e184f7dfe73eb8 100644
--- a/athena/wm/split_view_controller.cc
+++ b/athena/wm/split_view_controller.cc
@@ -67,6 +67,41 @@ bool SplitViewController::IsSplitViewModeActive() const {
   return state_ == ACTIVE;
 }
 
+void SplitViewController::ActivateSplitMode(aura::Window* left,
+                                            aura::Window* right) {
+  aura::Window::Windows windows = window_list_provider_->GetWindowList();
+  aura::Window::Windows::reverse_iterator iter = windows.rbegin();
+  if (state_ == ACTIVE) {
+    if (!left)
+      left = left_window_;
+    if (!right)
+      right = right_window_;
+  }
+
+  if (!left && iter != windows.rend()) {
+    left = *iter;
+    iter++;
+    if (left == right && iter != windows.rend()) {
+      left = *iter;
+      iter++;
+    }
+  }
+
+  if (!right && iter != windows.rend()) {
+    right = *iter;
+    iter++;
+    if (right == left && iter != windows.rend()) {
+      right = *iter;
+      iter++;
+    }
+  }
+
+  state_ = ACTIVE;
+  left_window_ = left;
+  right_window_ = right;
+  UpdateLayout(true);
+}
+
 void SplitViewController::UpdateLayout(bool animate) {
   if  (!left_window_)
     return;