add flag for future skia CL: moving Arithmetic mode to a native imagefilter

skia/config/SkUserConfig.h

 /*
  * Copyright (C) 2006 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
@@ skipping 210 matching lines @@
 #endif
 
 #ifndef    SK_IGNORE_GPU_DITHER
 #   define SK_IGNORE_GPU_DITHER
 #endif
 
 #ifndef    SK_SUPPORT_LEGACY_EVAL_CUBIC
 #   define SK_SUPPORT_LEGACY_EVAL_CUBIC
 #endif
 
+#ifndef    SK_SUPPORT_LEGACY_ARITHMETICMODE
+#   define SK_SUPPORT_LEGACY_ARITHMETICMODE
+#endif
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,
  *  or fall back to lazy runtime initialization. Chrome always wants the latter.
  */
 #define SK_ALLOW_STATIC_GLOBAL_INITIALIZERS 0
 
 /* This flag forces Skia not to use typographic metrics with GDI.
  */
 #define SK_GDI_ALWAYS_USE_TEXTMETRICS_FOR_FONT_METRICS
@@ skipping 16 matching lines @@
 
 // Updating to a correct SkPMColor lerp will require layout test rebaselines.
 #define SK_SUPPORT_LEGACY_BROKEN_LERP
 
 // Enabling the screenspace AA tessellating path renderer needs rebaselines.
 #define SK_DISABLE_SCREENSPACE_TESS_AA_PATH_RENDERER
 
 // ===== End Chrome-specific definitions =====
 
 #endif