Activate LayerImpl tree with sync+push instead of pointer swap

cc/layer_tree_host_impl_unittest.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/layer_tree_host_impl.h"
 
 #include <cmath>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 1503 matching lines @@
     // When we try to scroll a non-scrollable child layer, the scroll delta
     // should be applied to one of its ancestors if possible.
     gfx::Size surfaceSize(10, 10);
     gfx::Size contentSize(20, 20);
     scoped_ptr<LayerImpl> root = createScrollableLayer(1, contentSize);
     scoped_ptr<LayerImpl> child = createScrollableLayer(2, contentSize);
 
     child->setScrollable(false);
     root->addChild(child.Pass());
 
+    m_hostImpl->setViewportSize(surfaceSize, surfaceSize);
     m_hostImpl->activeTree()->SetRootLayer(root.Pass());
     m_hostImpl->activeTree()->DidBecomeActive();
-    m_hostImpl->setViewportSize(surfaceSize, surfaceSize);
     initializeRendererAndDrawFrame();
     {
         gfx::Vector2d scrollDelta(0, 4);
         EXPECT_EQ(m_hostImpl->scrollBegin(gfx::Point(5, 5), InputHandlerClient::Wheel), InputHandlerClient::ScrollStarted);
         m_hostImpl->scrollBy(gfx::Point(), scrollDelta);
         m_hostImpl->scrollEnd();
 
         scoped_ptr<ScrollAndScaleSet> scrollInfo = m_hostImpl->processScrollDeltas();
 
         // Only the root should have scrolled.
@@ skipping 3139 matching lines @@
         m_hostImpl->didDrawAllLayers(frame);
     }
 }
 
 INSTANTIATE_TEST_CASE_P(LayerTreeHostImplTests,
                         LayerTreeHostImplTest,
                         ::testing::Values(false, true));
 
 }  // namespace
 }  // namespace cc