Fix spurious 1px scrolling.

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/debug/trace_event.h"
 #include "base/debug/trace_event_argument.h"
 #include "base/json/json_reader.h"
 #include "base/strings/stringprintf.h"
@@ skipping 751 matching lines @@
 
   layer_tree_impl_->DidAnimateScrollOffset();
 }
 
 void LayerImpl::OnAnimationWaitingForDeletion() {}
 
 bool LayerImpl::IsActive() const {
   return layer_tree_impl_->IsActiveTree();
 }
 
 // TODO(wjmaclean) Convert so that bounds returns SizeF.

[danakj, 2014/08/20 17:00:25]

OT: A SizeF is going to be terrible for layers that are millions of pixels large
(think buildbot output), as the size of it will start to lose precision. Maybe
something else needs to be considered for this.

[aelias_OOO_until_Jul13, 2014/08/20 18:20:47]

The more serious issue is that layer bounds lack precision between DIPs.  This
is an underlying cause of this bug and also is a blocker for Android frontend UI
switching over to DIP coordinates.  I don't think the corner case of
8-million-pixel pages (I'm not sure even the buildbot pages are that long)
should be a greater concern, particularly since the floating point inaccuracy
would probably just cause minor rendering artifacts.  It doesn't seem worth
introducing fixed-point units just for this purpose either...

 gfx::Size LayerImpl::bounds() const {
-  return ToFlooredSize(temporary_impl_bounds_);
+  return ToCeiledSize(temporary_impl_bounds_);
 }
 
 void LayerImpl::SetBounds(const gfx::Size& bounds) {
   if (bounds_ == bounds)
     return;
 
   bounds_ = bounds;
   temporary_impl_bounds_ = bounds;
 
   ScrollbarParametersDidChange();
@@ skipping 750 matching lines @@
 }
 
 void LayerImpl::NotifyAnimationFinished(
     base::TimeTicks monotonic_time,
     Animation::TargetProperty target_property) {
   if (target_property == Animation::ScrollOffset)
     layer_tree_impl_->InputScrollAnimationFinished();
 }
 
 }  // namespace cc