Revert of Refactor unreliable hit test code to avoid owning_layers

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index f99af61cec2f13da73bae0bf17244f7fe536c3cf..132616ef303232e01822a4a4dd03ebb7dd1dbb86 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -162,10 +162,15 @@
     return false;
 
   auto* property_trees = child->layer_tree_impl()->property_trees();
+  auto ancestor_scroll_id =
+      property_trees->layer_id_to_scroll_node_index.find(ancestor->id());
+  if (ancestor_scroll_id == property_trees->layer_id_to_scroll_node_index.end())
+    return false;
+
   ScrollTree& scroll_tree = property_trees->scroll_tree;
   for (ScrollNode* scroll_node = scroll_tree.Node(child->scroll_tree_index());
        scroll_node; scroll_node = scroll_tree.parent(scroll_node)) {
-    if (scroll_node->id == ancestor->scroll_tree_index())
+    if (scroll_node->id == ancestor_scroll_id->second)
       return true;
   }
   return false;
@@ -2624,6 +2629,28 @@
   return active_tree_->LayerById(impl_scroll_node->owning_layer_id);
 }
 
+static bool IsClosestScrollAncestor(LayerImpl* child,
+                                    LayerImpl* scroll_ancestor) {
+  DCHECK(scroll_ancestor);
+  if (!child)
+    return false;
+
+  auto* property_trees = child->layer_tree_impl()->property_trees();
+  auto ancestor_scroll_id =
+      property_trees->layer_id_to_scroll_node_index.find(scroll_ancestor->id());
+  if (ancestor_scroll_id == property_trees->layer_id_to_scroll_node_index.end())
+    return false;
+
+  ScrollTree& scroll_tree = property_trees->scroll_tree;
+  ScrollNode* scroll_node = scroll_tree.Node(child->scroll_tree_index());
+  for (; scroll_tree.parent(scroll_node);
+       scroll_node = scroll_tree.parent(scroll_node)) {
+    if (scroll_node->scrollable)
+      return scroll_node->id == ancestor_scroll_id->second;
+  }
+  return false;
+}
+
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBeginImpl(
     ScrollState* scroll_state,
     LayerImpl* scrolling_layer_impl,
@@ -2697,7 +2724,11 @@
         active_tree_->FindLayerThatIsHitByPoint(device_viewport_point);
 
     if (layer_impl) {
-      if (!IsInitialScrollHitTestReliable(layer_impl, device_viewport_point)) {
+      LayerImpl* scroll_layer_impl =
+          active_tree_->FindFirstScrollingLayerOrScrollbarLayerThatIsHitByPoint(
+              device_viewport_point);
+      if (scroll_layer_impl &&
+          !IsClosestScrollAncestor(layer_impl, scroll_layer_impl)) {
         scroll_status.thread = SCROLL_UNKNOWN;
         scroll_status.main_thread_scrolling_reasons =
             MainThreadScrollingReason::kFailedHitTest;
@@ -2721,46 +2752,6 @@
   }
 
   return ScrollBeginImpl(scroll_state, scrolling_layer_impl, type);
-}
-
-// Some initial scroll tests are known to be unreliable and require falling
-// back to main thread scrolling.
-bool LayerTreeHostImpl::IsInitialScrollHitTestReliable(
-    LayerImpl* layer_impl,
-    const gfx::PointF& device_viewport_point) {
-  LayerImpl* first_scrolling_layer_or_drawn_scrollbar =
-      active_tree_->FindFirstScrollingLayerOrDrawnScrollbarThatIsHitByPoint(
-          device_viewport_point);
-  if (!first_scrolling_layer_or_drawn_scrollbar)
-    return true;
-
-  ScrollNode* closest_scroll_node = nullptr;
-  auto& scroll_tree = active_tree_->property_trees()->scroll_tree;
-  ScrollNode* scroll_node = scroll_tree.Node(layer_impl->scroll_tree_index());
-  for (; scroll_tree.parent(scroll_node);
-       scroll_node = scroll_tree.parent(scroll_node)) {
-    if (scroll_node->scrollable) {
-      closest_scroll_node = scroll_node;
-      break;
-    }
-  }
-  if (!closest_scroll_node)
-    return false;
-
-  // If |first_scrolling_layer_or_drawn_scrollbar| is scrollable, it will
-  // create a scroll node. If this scroll node corresponds to first scrollable
-  // ancestor along the scroll tree for |layer_impl|, the hit test has not
-  // escaped to other areas of the scroll tree and is reliable.
-  if (first_scrolling_layer_or_drawn_scrollbar->scrollable()) {
-    return closest_scroll_node->id ==
-           first_scrolling_layer_or_drawn_scrollbar->scroll_tree_index();
-  }
-
-  // If |first_scrolling_layer_or_drawn_scrollbar| is not scrollable, it must
-  // be a drawn scrollbar. These hit tests require falling back to main-thread
-  // scrolling.
-  DCHECK(first_scrolling_layer_or_drawn_scrollbar->IsDrawnScrollbar());
-  return false;
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimatedBegin(