add flag for future Skia CL

See https://skia-review.googlesource.com/c/21372/

Bug: 
Change-Id: I2034583f24a1de64474f63a4db123737321a5c4f
Reviewed-on: https://chromium-review.googlesource.com/558766
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484011}

skia/config/SkUserConfig.h

@@ -216,6 +216,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_TILED_BITMAPS
 #endif
 
+#ifndef SK_SUPPORT_NONSTD_BLENDMODES
+#define SK_SUPPORT_NONSTD_BLENDMODES
+#endif
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,