add SK_SUPPORT_LEGACY_SCALAR_XFERMODES to stage skia speedup for xfermodes

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 235 matching lines @@
 #endif
 
 #ifndef    SK_LEGACY_DRAWPICTURECALLBACK
 #   define SK_LEGACY_DRAWPICTURECALLBACK
 #endif
 
 #ifndef    SK_SUPPORT_LEGACY_GETDEVICE
 #   define SK_SUPPORT_LEGACY_GETDEVICE
 #endif
 
+#ifndef    SK_SUPPORT_LEGACY_SCALAR_XFERMODES
+#   define SK_SUPPORT_LEGACY_SCALAR_XFERMODES
+#endif
+
 #ifndef    SK_SUPPORT_LEGACY_PUBLIC_IMAGEINFO_FIELDS
 #   define SK_SUPPORT_LEGACY_PUBLIC_IMAGEINFO_FIELDS
 #endif
 
 #ifndef    SK_SUPPORT_LEGACY_SCALAR_MAPPOINTS
 #   define SK_SUPPORT_LEGACY_SCALAR_MAPPOINTS
 #endif
 
 #ifndef    SK_IGNORE_ETC1_SUPPORT
 #   define SK_IGNORE_ETC1_SUPPORT
@@ skipping 40 matching lines @@
 #define SK_USE_DISCARDABLE_SCALEDIMAGECACHE
 #define SK_WILL_NEVER_DRAW_PERSPECTIVE_TEXT
 
 #define SK_ATTR_DEPRECATED          SK_NOTHING_ARG1
 #define SK_ENABLE_INST_COUNT        0
 #define GR_GL_CUSTOM_SETUP_HEADER   "GrGLConfig_chrome.h"
 
 // ===== End Chrome-specific definitions =====
 
 #endif