Add guard for upcoming Skia change

SK_SUPPORT_LEGACY_A8_MASKBLITTER, for
https://skia-review.googlesource.com/c/skia/+/121642

Change-Id: Id47e95cb764c526864f3af8350a546254ee1c54d
Reviewed-on: https://chromium-review.googlesource.com/1016670Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551785}

skia/config/SkUserConfig.h

@@ -199,6 +199,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_DISABLE_RENDER_TARGET_SORTING
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_A8_MASKBLITTER
+#define SK_SUPPORT_LEGACY_A8_MASKBLITTER
+#endif
+
 #ifndef SK_SUPPORT_LEGACY_TILED_BITMAPS
 #define SK_SUPPORT_LEGACY_TILED_BITMAPS
 #endif