split view: Remove obsolete DCHECK

Near the end of SplitViewHighlightView::OnWindowDraggingStateChanged,
there is a DCHECK that the previous window dragging state is kNoDrag,
when actually it is expected that the previous window dragging state
also can be kOtherDisplay. I should have removed this DCHECK in
https://chromium-review.googlesource.com/c/chromium/src/+/1977155

Bug: 961170
Change-Id: Ifdd27ae5e610f05e754c401e7805798df4a95098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981648Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728008}

ash/wm/splitview/split_view_highlight_view.cc

@@ -252,10 +252,8 @@ void SplitViewHighlightView::OnWindowDraggingStateChanged(
     return;
   }
 
-  // The drag just started, and not in a snap area. If |previews_only|, there is
-  // nothing to do. Else fade in.
-  DCHECK_EQ(SplitViewDragIndicators::WindowDraggingState::kNoDrag,
-            previous_window_dragging_state);
+  // The drag just started or came in from another display, and is not currently
+  // in a snap area. If |previews_only|, there is nothing to do. Else fade in.
   DCHECK_EQ(0.f, layer()->GetTargetOpacity());
   if (!previews_only) {
     DoSplitviewOpacityAnimation(layer(), SPLITVIEW_ANIMATION_HIGHLIGHT_FADE_IN);