add flag for future Skia CL

needed for https://skia-review.googlesource.com/c/skia/+/113940

Change-Id: I4db6feb9e518cdacbaba10fcbd3e790927f3d4a3
Reviewed-on: https://chromium-review.googlesource.com/959531Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542723}

skia/config/SkUserConfig.h

@@ -213,6 +213,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_LEGACY_GPU_PIXEL_OPS
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_BLURMASKFILTER
+#define SK_SUPPORT_LEGACY_BLURMASKFILTER
+#endif
+
 #ifndef SK_SUPPORT_LEGACY_YUV_COLORSPACE
 #define SK_SUPPORT_LEGACY_YUV_COLORSPACE
 #endif