cc: Make impl-thread property trees handle fixed-position layers

cc/layers/layer_position_constraint_unittest.cc

 // Copyright 2013 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_position_constraint.h"
 
 #include <vector>
 
 #include "cc/layers/layer.h"
 #include "cc/layers/layer_impl.h"
@@ skipping 957 matching lines @@
   // would still have to compensate with respect to its container.
 
   // Add one more layer to the hierarchy for this test.
   scoped_refptr<Layer> great_great_grand_child =
       make_scoped_refptr(new LayerWithForcedDrawsContent(layer_settings_));
   great_grand_child_->AddChild(great_great_grand_child);
 
   child_->SetIsContainerForFixedPositionLayers(true);
   grand_child_->SetPositionConstraint(fixed_to_top_left_);
   great_grand_child_->SetIsContainerForFixedPositionLayers(true);
+  great_grand_child_->SetScrollClipLayerId(root_->id());

[ajuma, 2015/05/29 17:47:08]

This test scrolls great_grand_child_, so it should be scrollable (so that a
transform node gets created).

[enne (OOO), 2015/05/29 20:07:53]

Is that something that can be DCHECK'd?

[ajuma, 2015/05/29 21:41:07]

Added a DCHECK to LayerImpl::SetScrollDelta.

   great_great_grand_child->SetPositionConstraint(fixed_to_top_left_);
 
   CommitAndUpdateImplPointers();
 
   LayerImpl* container1 = child_impl_;
   LayerImpl* fixed_to_container1 = grand_child_impl_;
   LayerImpl* container2 = great_grand_child_impl_;
   LayerImpl* fixed_to_container2 = container2->children()[0];
 
   container1->SetScrollDelta(gfx::Vector2d(0, 15));
@@ skipping 21 matching lines @@
 
   EXPECT_TRANSFORMATION_MATRIX_EQ(expected_container2_transform,
                                   container2->draw_transform());
 
   EXPECT_TRANSFORMATION_MATRIX_EQ(expected_fixed_to_container2_transform,
                                   fixed_to_container2->draw_transform());
 }
 
 }  // namespace
 }  // namespace cc