Make the scroll_delta flooring behavior the same as before SyncedProperty change

cc/layers/layer_impl.cc

 // Copyright 2012 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/layers/layer_impl.h"
 
 #include "base/json/json_reader.h"
 #include "base/strings/stringprintf.h"
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
@@ skipping 1065 matching lines @@
 }
 
 void LayerImpl::PushScrollOffsetFromMainThread(
     const gfx::ScrollOffset& scroll_offset) {
   PushScrollOffset(&scroll_offset);
 }
 
 gfx::ScrollOffset LayerImpl::PullDeltaForMainThread() {
   RefreshFromScrollDelegate();
 
-  // TODO(aelias): Remove all this temporary flooring machinery when Blink
-  // fully supports fractional scrolls.
-  gfx::ScrollOffset current = CurrentScrollOffset();
-  gfx::ScrollOffset floored(floor(current.x()), floor(current.y()));
-  scroll_offset_->SetCurrent(floored);
+  // TODO(aelias, miletus): Remove all this temporary flooring machinery when
+  // Blink fully supports fractional scrolls.
+  gfx::ScrollOffset current_offset = CurrentScrollOffset();
+  gfx::Vector2dF current_delta = ScrollDelta();
+  gfx::Vector2dF floored_delta(floor(current_delta.x()),
+                               floor(current_delta.y()));
+  gfx::Vector2dF diff_delta = floored_delta - current_delta;
+  gfx::ScrollOffset tmp_offset = ScrollOffsetWithDelta(current_offset,
+                                                       diff_delta);
+  scroll_offset_->SetCurrent(tmp_offset);
   gfx::ScrollOffset delta = scroll_offset_->PullDeltaForMainThread();
-  scroll_offset_->SetCurrent(current);
+  scroll_offset_->SetCurrent(current_offset);
   return delta;
 }
 
 void LayerImpl::RefreshFromScrollDelegate() {
   if (scroll_offset_delegate_) {
     SetCurrentScrollOffset(
         gfx::ScrollOffset(scroll_offset_delegate_->GetCurrentScrollOffset()));
   }
 }
 
@@ skipping 430 matching lines @@
   SetNeedsPushProperties();
   layer_tree_impl()->set_needs_update_draw_properties();
   if (should_have_render_surface) {
     render_surface_ = make_scoped_ptr(new RenderSurfaceImpl(this));
     return;
   }
   render_surface_.reset();
 }
 
 }  // namespace cc