Revert "Unregister cc scrollbar when registering new one"

This reverts commit 8448024f.

Reason for revert: This likely introduced a container-overflow: crbug.com/742520

Original change's description:
> Unregister cc scrollbar when registering new one
> 
> We previously DCHECKed that we could never register a new scrollbar
> against a scroll layer that already has a scrollbar. i.e. We expected
> that the old scrollbar would be unregistered prior to registering the
> new one.
> 
> RootScroller can break this assumption on Android. On Android, the
> global rootScroller (i.e. OuterViewport) is prevented from creating
> its own scrollbars, instead inheriting the VisualViewport-created
> scrollbars. In other words, the VisualViewport creates a pair of
> scrollbars and they get re-assigned to whatever the current outer
> viewport is.
> 
> This means that when we change the global rootScroller (for context see
> core/page/scrolling/README.md), the previous rootScroller now creates
> its own scrollbars and the VisualViewport scrollbars get registered to
> the new rootScroller. This then creates a race between moving the
> VisualViewport scrollbar registration and new scrollbar registration on
> the old rootScroller.
> 
> Long-term, the outer viewport shouldn't own the scrollbars but for now
> it doesn't hurt to just unregister the scrollbar if we notice the
> scroller already has scrollbars registered.
> 
> Bug: 739738
> Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
> Change-Id: I99e8a95881e7dad19c0318a661da80f441b97b19
> Reviewed-on: https://chromium-review.googlesource.com/562018
> Reviewed-by: Weiliang Chen <weiliangc@chromium.org>
> Commit-Queue: David Bokan <bokan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#486086}

TBR=bokan@chromium.org,weiliangc@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 739738, 742520, 742678
Change-Id: I1134e17dcb04a3a2cc50f07ecbc0fc79cc74f344
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/574547
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487082}

cc/trees/layer_tree_impl.cc

@@ -1656,20 +1656,15 @@ void LayerTreeImpl::RegisterScrollbar(ScrollbarLayerImplBase* scrollbar_layer) {
     return;
 
   auto& scrollbar_ids = element_id_to_scrollbar_layer_ids_[scroll_element_id];
-  int& scrollbar_layer_id = scrollbar_layer->orientation() == HORIZONTAL
-                                ? scrollbar_ids.horizontal
-                                : scrollbar_ids.vertical;
-
-  // We used to DCHECK this was not the case but this can occur on Android: as
-  // the visual viewport supplies scrollbars for the outer viewport, if the
-  // outer viewport is changed, we race between updating the visual viewport
-  // scrollbars and registering new scrollbars on the old outer viewport. It'd
-  // be nice if we could fix this to be cleaner but its harmless to just
-  // unregister here.
-  if (scrollbar_layer_id != Layer::INVALID_ID)
-    UnregisterScrollbar(scrollbar_layer);
-
-  scrollbar_layer_id = scrollbar_layer->id();
+  if (scrollbar_layer->orientation() == HORIZONTAL) {
+    DCHECK_EQ(scrollbar_ids.horizontal, Layer::INVALID_ID)
+        << "Existing scrollbar should have been unregistered.";
+    scrollbar_ids.horizontal = scrollbar_layer->id();
+  } else {
+    DCHECK_EQ(scrollbar_ids.vertical, Layer::INVALID_ID)
+        << "Existing scrollbar should have been unregistered.";
+    scrollbar_ids.vertical = scrollbar_layer->id();
+  }
 
   if (IsActiveTree() && scrollbar_layer->is_overlay_scrollbar() &&
       scrollbar_layer->GetScrollbarAnimator() !=