Adds scroll position/scale emulation to DevTools.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2530 matching lines @@
     const gfx::PointF& device_viewport_point,
     InputHandler::ScrollInputType type,
     LayerImpl* layer_impl,
     bool* scroll_on_main_thread,
     uint32_t* main_thread_scrolling_reasons) const {
   DCHECK(scroll_on_main_thread);
   DCHECK(main_thread_scrolling_reasons);
   *main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
 
+  // Falling back to the root scroll layer ensures generation of root overscroll
+  // notifications. The inner viewport layer represents the viewport during
+  // scrolling.
+  if (!layer_impl)
+    layer_impl = InnerViewportScrollLayer();
+
   // Walk up the hierarchy and look for a scrollable layer.
   LayerImpl* potentially_scrolling_layer_impl = NULL;
   if (layer_impl) {
     ScrollTree& 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)) {
       // The content layer can also block attempts to scroll outside the main
       // thread.
       ScrollStatus status =
@@ skipping 12 matching lines @@
         return NULL;
       }
 
       if (status.thread == InputHandler::SCROLL_ON_IMPL_THREAD &&
           !potentially_scrolling_layer_impl) {
         potentially_scrolling_layer_impl =
             active_tree_->LayerById(scroll_node->owner_id);
       }
     }
   }
-  // Falling back to the root scroll layer ensures generation of root overscroll
-  // notifications while preventing scroll updates from being unintentionally
-  // forwarded to the main thread. The inner viewport layer represents the
-  // viewport during scrolling.
-  if (!potentially_scrolling_layer_impl)
-    potentially_scrolling_layer_impl = InnerViewportScrollLayer();
 
   // The inner viewport layer represents the viewport.
   if (potentially_scrolling_layer_impl == OuterViewportScrollLayer())
     potentially_scrolling_layer_impl = InnerViewportScrollLayer();
 
   return potentially_scrolling_layer_impl;
 }
 
 // Similar to LayerImpl::HasAncestor, but walks up the scroll parents.
 static bool HasScrollAncestor(LayerImpl* child, LayerImpl* scroll_ancestor) {
@@ skipping 1409 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc