Prepare to use new small mask blur.

Change-Id: I2fa873e63695195ee01d0d67c61430a4d0afb565
Reviewed-on: https://chromium-review.googlesource.com/764629Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Herb Derby <herb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515756}

skia/config/SkUserConfig.h

@@ -224,6 +224,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_DELTA_AA
 #endif
 
+#ifndef SK_USE_LEGACY_INTERP_BLUR
+#define SK_USE_LEGACY_INTERP_BLUR
+#endif
+
 #ifndef SK_SUPPORT_LEGACY_SMALLRECT_AA
 #define SK_SUPPORT_LEGACY_SMALLRECT_AA
 #endif