[CompositorThreadedScrollbarScrolling] Cleanup for scroll chaining.

This CL deletes the workaround that exists in scrollbar_controller
which was added for preventing scroll chaining. This is no longer
needed since crbug.com/996537 has been fixed and ScrollEndImpl doesn't
get called before ScrollAnimatedBegin anymore. The scrolling_node in
LayerTreeHostImpl::ScrollBegin can be determined by the information
contained in the synthetic WebGestureEvent's ScrollState.

Bug: 1039976
Change-Id: I93b818cd57a25eb3355ce07c2d1c39a51cec3d49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1991944Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#729794}

cc/input/scrollbar_controller.cc

@@ -123,20 +123,6 @@ InputHandlerPointerResult ScrollbarController::HandlePointerDown(
         ->PostDelayedTask(FROM_HERE, cancelable_autoscroll_task_->callback(),
                           kInitialAutoscrollTimerDelay);
   }
-  // Since compositor threaded scrollbar scrolling is gesture-based, the
-  // following code is needed to prevent scroll chaining.
-  // TODO(savella): This should be solved by not chaining element-id based
-  // scrolls
-  ScrollTree& scroll_tree =
-      layer_tree_host_impl_->active_tree()->property_trees()->scroll_tree;
-  ScrollNode* scroll_node =
-      scroll_tree.FindNodeFromElementId(scrollbar->scroll_element_id());
-  gfx::Vector2dF delta = gfx::Vector2dF(scroll_result.scroll_offset.x(),
-                                        scroll_result.scroll_offset.y());
-  if (scroll_node != nullptr && layer_tree_host_impl_->ComputeScrollDelta(
-                                    *scroll_node, delta) == gfx::Vector2dF()) {
-    scroll_result.scroll_offset = gfx::ScrollOffset();
-  }
   return scroll_result;
 }