cc: Enable composited border-radius scrolling.

cc/trees/layer_tree_settings.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_SETTINGS_H_
 #define CC_TREES_LAYER_TREE_SETTINGS_H_
 
 #include <stddef.h>
 
 #include <vector>
@@ skipping 73 matching lines @@
   ManagedMemoryPolicy gpu_memory_policy;
   ManagedMemoryPolicy software_memory_policy;
   size_t decoded_image_cache_budget_bytes = 128 * 1024 * 1024;
   size_t decoded_image_working_set_budget_bytes = 128 * 1024 * 1024;
   int max_preraster_distance_in_screen_pixels = 1000;
 
   bool enable_color_correct_rasterization = false;
 
   // TODO(sunxd): remove this flag when filter demoting and aa of mask layers
   // are implemented.
-  bool enable_mask_tiling = false;
+  bool enable_mask_tiling = true;

[chrishtr, 2017/05/19 00:28:15]

You could also turn this on without border-radius scrolling right?

 
   // If set to true, the compositor may selectively defer image decodes to the
   // Image Decode Service and raster tiles without images until the decode is
   // ready.
   bool enable_checker_imaging = false;
 
   LayerTreeDebugState initial_debug_state;
 
   // Indicates that the LayerTreeHost should defer commits unless it has a valid
   // LocalSurfaceId set.
   bool enable_surface_synchronization = false;
 
   // Indicates the case when a sub-frame gets its own LayerTree because it's
   // rendered in a different process from its ancestor frames.
   bool is_layer_tree_for_subframe = false;
 
   // Determines whether we disallow non-exact matches when finding resources
   // in ResourcePool. Only used for layout or pixel tests, as non-deterministic
   // resource sizes can lead to floating point error and noise in these tests.
   bool disallow_non_exact_resource_reuse = false;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_SETTINGS_H_