Round the scroll offset synced back to main instead of flooring.

cc/trees/property_tree.cc

 // Copyright 2014 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 <stddef.h>
 
 #include <set>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 1202 matching lines @@
 
 const gfx::ScrollOffset ScrollTree::current_scroll_offset(int layer_id) const {
   return synced_scroll_offset(layer_id)
              ? synced_scroll_offset(layer_id)->Current(
                    property_trees()->is_active)
              : gfx::ScrollOffset();
 }
 
 gfx::ScrollOffset ScrollTree::PullDeltaForMainThread(
     SyncedScrollOffset* scroll_offset) {
-  // TODO(miletus): Remove all this temporary flooring machinery when
-  // Blink fully supports fractional scrolls.
+  // TODO(flackr): We should pass the fractional scroll deltas when Blink fully
+  // supports fractional scrolls.
+  // TODO(flackr): We should ideally round the fractional scrolls in the same
+  // direction as the scroll will be snapped but for common cases this is
+  // equivalent to rounding to the nearest integer offset.
   gfx::ScrollOffset current_offset =
       scroll_offset->Current(property_trees()->is_active);
-  gfx::ScrollOffset current_delta = property_trees()->is_active
-                                        ? scroll_offset->Delta()
-                                        : scroll_offset->PendingDelta().get();
-  gfx::ScrollOffset floored_delta(floor(current_delta.x()),
-                                  floor(current_delta.y()));
-  gfx::ScrollOffset diff_delta = floored_delta - current_delta;
-  gfx::ScrollOffset tmp_offset = current_offset + diff_delta;
-  scroll_offset->SetCurrent(tmp_offset);
+  gfx::ScrollOffset rounded_offset =
+      gfx::ScrollOffset(roundf(current_offset.x()), roundf(current_offset.y()));
+  scroll_offset->SetCurrent(rounded_offset);
   gfx::ScrollOffset delta = scroll_offset->PullDeltaForMainThread();
   scroll_offset->SetCurrent(current_offset);
   return delta;
 }
 
 void ScrollTree::CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
                                      int inner_viewport_layer_id) {
   for (auto map_entry : layer_id_to_scroll_offset_map_) {
     gfx::ScrollOffset scroll_delta =
         PullDeltaForMainThread(map_entry.second.get());
@@ skipping 733 matching lines @@
 
   const EffectNode* effect_node = effect_tree.Node(effect_id);
 
   bool success = GetFromTarget(transform_id, effect_id, transform);
   transform->Scale(effect_node->surface_contents_scale.x(),
                    effect_node->surface_contents_scale.y());
   return success;
 }
 
 }  // namespace cc