Fix the crash after clamshell -> tablet transition in overview mode.

This CL just reverted some changes that were made in
https://chromium-review.googlesource.com/c/chromium/src/+/1658955. In
that CL, we changed the clamshell <-> tablet transition when clamshell
split view mode is enabled, however, we should keep the old behavior
unchanged if the feature is not enabled, i.e., overview should be ended
if it's active before the transition. Otherwise, it will cause a nullptr
dereference crash since |split_view_drag_indicators_| is not created in
clamshell overview and will be used in tablet overview.

Bug: 982507
Change-Id: I238fe9472648a446cff4ab992150658c228714dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1705474
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima (Slow - on/off site) <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679306}
bool IsTabDraggingSourceWindow(aura::Window* window) {
  if (!window)
    return false;

  MruWindowTracker::WindowList window_list =
      Shell::Get()->mru_window_tracker()->BuildMruWindowList(kActiveDesk);
  if (window_list.empty())
    return false;

  aura::Window* dragged_window = nullptr;
  for (auto* window : window_list) {
    if (wm::IsDraggingTabs(window)) {
      dragged_window = window;
      break;
    }
  }
  if (!dragged_window)
    return false;

  return dragged_window->GetProperty(ash::kTabDraggingSourceWindowKey) ==
         window;
}

bool IsTabDraggingSourceWindow(aura::Window* window) {
  if (!window)
    return false;

  MruWindowTracker::WindowList window_list =
      Shell::Get()->mru_window_tracker()->BuildMruWindowList(kActiveDesk);
  if (window_list.empty())
    return false;

  aura::Window* dragged_window = nullptr;
  for (auto* window : window_list) {
    if (wm::IsDraggingTabs(window)) {
      dragged_window = window;
      break;
    }
  }
  if (!dragged_window)
    return false;

  return dragged_window->GetProperty(ash::kTabDraggingSourceWindowKey) ==
         window;
}
