add flag to stage mipmap change

prereq for https://codereview.chromium.org/916103008

BUG=

Review URL: https://codereview.chromium.org/937233002

Cr-Commit-Position: refs/heads/master@{#317145}

skia/config/SkUserConfig.h

@@ -253,6 +253,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #   define SK_IGNORE_ETC1_SUPPORT
 #endif
 
+#ifndef    SK_SUPPORT_LEGACY_MIPMAP_EFFECTIVE_SCALE
+#   define SK_SUPPORT_LEGACY_MIPMAP_EFFECTIVE_SCALE
+#endif
+
 #ifndef    SK_IGNORE_GPU_DITHER
 #   define SK_IGNORE_GPU_DITHER
 #endif