overview: Disable entering overview while dragging split view divider

You shall not be able to enter overview by pressing the overview button
with one finger, while dragging the split view divider with another
finger. See the video linked on Monorail.

Test: manual
Change-Id: I779e20fcc951d44355ee4c05d71e5bac1c83024f
Fixed: 1051758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055735Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741229}

ash/wm/overview/overview_controller.cc

@@ -447,9 +447,11 @@ void OverviewController::ToggleOverview(
 }
 
 bool OverviewController::CanEnterOverview() {
-  // Prevent toggling overview during the split view divider snap animation.
-  if (SplitViewController::Get(Shell::GetPrimaryRootWindow())
-          ->IsDividerAnimating()) {
+  // Prevent entering overview while the divider is dragged or animated.
+  SplitViewController* split_view_controller =
+      SplitViewController::Get(Shell::GetPrimaryRootWindow());
+  if (split_view_controller->is_resizing() ||
+      split_view_controller->IsDividerAnimating()) {
     return false;
   }