overview: Do not end overview when split view starts with an ARC window

The present CL addresses blatantly broken behavior when entering split
view mode by snapping an ARC window.

Bug: b/143499564
Change-Id: I777f03a356301d2be507fc85b74499ecc3a57173
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893585Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711353}

ash/wm/overview/overview_session.cc

@@ -646,9 +646,15 @@ void OverviewSession::OnWindowActivating(
 
   // Do not cancel overview mode if the window activation happens when split
   // view mode is also active. SplitViewController will do the right thing to
-  // handle the window activation change.
-  if (SplitViewController::Get(gained_active)->InSplitViewMode())
+  // handle the window activation change. Check for split view mode without
+  // using |SplitViewController::state_| which is updated asynchronously when
+  // snapping an ARC window.
+  SplitViewController* split_view_controller =
+      SplitViewController::Get(gained_active);
+  if (split_view_controller->left_window() ||
+      split_view_controller->right_window()) {
     return;
+  }
 
   // Do not cancel overview mode if the window activation was caused while
   // dragging overview mode offscreen.