add flag for future Skia CL

See https://skia-review.googlesource.com/c/24260/

Bug: 
Change-Id: I82348c8d51e810a6519fe48f79a55ce3407fc76b
Reviewed-on: https://chromium-review.googlesource.com/575831
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487414}

skia/config/SkUserConfig.h

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