cc: Fix pixeltest expectations for the waterfall.

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"
@@ skipping 146 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 inner edge is antialiased and gets off-by-one differences.
-  float percentage_pixels_large_error = 0.42f;
+  // 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