add flag for future Skia CL

See https://skia-review.googlesource.com/c/21739/

NOTRY=True

Bug: 
Change-Id: I070f26e88d0ad3b4925c082b84ef1f9a148212cf
Reviewed-on: https://chromium-review.googlesource.com/565767Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485406}

skia/config/SkUserConfig.h

@@ -224,6 +224,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_BILERP_IGNORING_HACK
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_MASK_BLUR
+#define SK_SUPPORT_LEGACY_MASK_BLUR
+#endif
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,