Clear the scaled bitmap before drawing into it for huge UIResources.

cc/trees/layer_tree_host_pixeltest_scrollbars.cc

 // Copyright 2016 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 <stddef.h>
 
 #include "base/memory/ptr_util.h"
 #include "build/build_config.h"
 #include "cc/input/scrollbar.h"
 #include "cc/layers/painted_scrollbar_layer.h"
 #include "cc/layers/solid_color_layer.h"
 #include "cc/test/layer_tree_pixel_test.h"
-#include "cc/test/pixel_comparator.h"
 #include "cc/test/test_in_process_context_provider.h"
 #include "gpu/command_buffer/client/gles2_interface.h"
 
 #if !defined(OS_ANDROID)
 
 namespace cc {
 namespace {
 
 class LayerTreeHostScrollbarsPixelTest : public LayerTreePixelTest {
  protected:
@@ skipping 133 matching lines @@
 
   // Let's show the bottom right of the layer, so we know the texture wasn't
   // just cut off.
   layer->SetPosition(
       gfx::PointF(-10.f * scale + 400.f, -400.f * scale + 400.f));
 
   gfx::Transform scale_transform;
   scale_transform.Scale(scale, scale);
   layer->SetTransform(scale_transform);
 
-  // The edges are antialiased and get off-by-one differences.
-  float percentage_pixels_large_error = 0.80125f;
-  float percentage_pixels_small_error = 0.f;
-  float average_error_allowed_in_bad_pixels = 1.f;
-  int large_error_allowed = 1;
-  int small_error_allowed = 0;
-  pixel_comparator_.reset(new FuzzyPixelComparator(
-      true,  // discard_alpha
-      percentage_pixels_large_error, percentage_pixels_small_error,
-      average_error_allowed_in_bad_pixels, large_error_allowed,
-      small_error_allowed));
-
   RunPixelTest(PIXEL_TEST_GL, background,
                base::FilePath(FILE_PATH_LITERAL("spiral_64_scale.png")));
 }
 
 }  // namespace
 }  // namespace cc
 
 #endif  // OS_ANDROID