Don't clear render surfaces unnecessarily.

cc/trees/layer_tree_host_common_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/trees/layer_tree_host_common.h"
 
 #include <set>
 
 #include "cc/animation/layer_animation_controller.h"
 #include "cc/animation/transform_operations.h"
@@ skipping 1438 matching lines @@
       gfx::Transform(),
       &render_surface_layer_list);
   LayerTreeHostCommon::CalculateDrawProperties(&inputs);
 
   // The child layer's content is entirely outside the parent's clip rect, so
   // the intermediate render surface should not be listed here, even if it was
   // forced to be created. Render surfaces without children or visible content
   // are unexpected at draw time (e.g. we might try to create a content texture
   // of size 0).
   ASSERT_TRUE(parent->render_surface());
-  ASSERT_FALSE(render_surface1->render_surface());
   EXPECT_EQ(1U, render_surface_layer_list.size());
 }
 
 TEST_F(LayerTreeHostCommonTest, RenderSurfaceListForTransparentChild) {
   scoped_refptr<Layer> parent = Layer::Create();
   scoped_refptr<Layer> render_surface1 = Layer::Create();
   scoped_refptr<LayerWithForcedDrawsContent> child =
       make_scoped_refptr(new LayerWithForcedDrawsContent());
 
   scoped_ptr<FakeLayerTreeHost> host = FakeLayerTreeHost::Create();
@@ skipping 689 matching lines @@
   grand_child4->SetForceRenderSurface(true);
 
   RenderSurfaceLayerList render_surface_layer_list;
   LayerTreeHostCommon::CalcDrawPropsMainInputsForTesting inputs(
       parent.get(), parent->bounds(), &render_surface_layer_list);
   inputs.can_adjust_raster_scales = true;
   LayerTreeHostCommon::CalculateDrawProperties(&inputs);
   ASSERT_TRUE(grand_child1->render_surface());
   ASSERT_TRUE(grand_child2->render_surface());
   ASSERT_TRUE(grand_child3->render_surface());
-  // Because grand_child4 is entirely clipped, it is expected to not have a
-  // render surface.
-  EXPECT_FALSE(grand_child4->render_surface());
 
   // Surfaces are clipped by their parent, but un-affected by the owning layer's
   // masksToBounds.
   EXPECT_RECT_EQ(gfx::Rect(0, 0, 20, 20),
                  grand_child1->render_surface()->clip_rect());
   EXPECT_RECT_EQ(gfx::Rect(0, 0, 20, 20),
                  grand_child2->render_surface()->clip_rect());
   EXPECT_RECT_EQ(gfx::Rect(0, 0, 20, 20),
                  grand_child3->render_surface()->clip_rect());
 }
@@ skipping 7892 matching lines @@
 
   // |grand_parent| has a transform that's neither a translation nor a scale.
   EXPECT_EQ(0.f, grand_parent->last_maximum_animation_contents_scale());
   EXPECT_EQ(0.f, parent_raw->last_maximum_animation_contents_scale());
   EXPECT_EQ(0.f, child_raw->last_maximum_animation_contents_scale());
   EXPECT_EQ(0.f, grand_child_raw->last_maximum_animation_contents_scale());
 }
 
 }  // namespace
 }  // namespace cc